[SDK] Fix circularity issues with overlay shim headers.
[SDK] Fix circularity issues with overlay shim headers.

Loading a Clang module eagerly brings in overlays for anything it re-exports,
but this is a problem for these new "shim header" modules, which generally
import the underlying module for an overlay and are in turn imported by the
overlay. That means that when we try to import an overlay, we'll end up with
a circular reference before it's done loading all its dependencies. Break the
cycle by not exporting anything from these modules, which are mostly just an
implementation detail anyway.

hooman/swift,natecook1000/swift,brentdax/swift,hooman/swift,alblue/swift,jckarter/swift,tjw/swift,hooman/swift,atrick/swift,milseman/swift,stephentyrone/swift,lorentey/swift,apple/swift,tardieu/swift,codestergit/swift,amraboelela/swift,ben-ng/swift,therealbnut/swift,sschiau/swift,amraboelela/swift,devincoughlin/swift,karwa/swift,roambotics/swift,milseman/swift,jckarter/swift,jmgc/swift,natecook1000/swift,frootloops/swift,JGiola/swift,xedin/swift,parkera/swift,milseman/swift,codestergit/swift,gregomni/swift,bitjammer/swift,xedin/swift,uasys/swift,ahoppen/swift,harlanhaskins/swift,rudkx/swift,allevato/swift,gribozavr/swift,therealbnut/swift,gmilos/swift,apple/swift,therealbnut/swift,tkremenek/swift,kstaring/swift,gmilos/swift,arvedviehweger/swift,gmilos/swift,Jnosh/swift,tinysun212/swift-windows,tkremenek/swift,Jnosh/swift,airspeedswift/swift,nathawes/swift,tjw/swift,bitjammer/swift,zisko/swift,alblue/swift,djwbrown/swift,devincoughlin/swift,brentdax/swift,parkera/swift,ben-ng/swift,CodaFi/swift,gottesmm/swift,shajrawi/swift,deyton/swift,shahmishal/swift,OscarSwanros/swift,tinysun212/swift-windows,devincoughlin/swift,sschiau/swift,gottesmm/swift,manavgabhawala/swift,gottesmm/swift,uasys/swift,hughbe/swift,stephentyrone/swift,jmgc/swift,gribozavr/swift,karwa/swift,allevato/swift,natecook1000/swift,natecook1000/swift,practicalswift/swift,ben-ng/swift,benlangmuir/swift,milseman/swift,calebd/swift,felix91gr/swift,shahmishal/swift,practicalswift/swift,jmgc/swift,jtbandes/swift,OscarSwanros/swift,allevato/swift,aschwaighofer/swift,shahmishal/swift,gottesmm/swift,kperryua/swift,xwu/swift,lorentey/swift,stephentyrone/swift,jopamer/swift,xedin/swift,gribozavr/swift,airspeedswift/swift,gregomni/swift,parkera/swift,jmgc/swift,shajrawi/swift,danielmartin/swift,jckarter/swift,gmilos/swift,shajrawi/swift,swiftix/swift,lorentey/swift,glessard/swift,deyton/swift,alblue/swift,frootloops/swift,gribozavr/swift,hooman/swift,kstaring/swift,deyton/swift,swiftix/swift,ben-ng/swift,return/swift,xwu/swift,bitjammer/swift,jtbandes/swift,frootloops/swift,codestergit/swift,manavgabhawala/swift,therealbnut/swift,gregomni/swift,calebd/swift,airspeedswift/swift,karwa/swift,xedin/swift,xwu/swift,JGiola/swift,JaSpa/swift,felix91gr/swift,parkera/swift,stephentyrone/swift,sschiau/swift,calebd/swift,danielmartin/swift,alblue/swift,codestergit/swift,tinysun212/swift-windows,manavgabhawala/swift,austinzheng/swift,gmilos/swift,brentdax/swift,gregomni/swift,austinzheng/swift,kstaring/swift,tinysun212/swift-windows,gregomni/swift,felix91gr/swift,kperryua/swift,gribozavr/swift,harlanhaskins/swift,rudkx/swift,hughbe/swift,zisko/swift,glessard/swift,JaSpa/swift,ahoppen/swift,aschwaighofer/swift,lorentey/swift,danielmartin/swift,devincoughlin/swift,danielmartin/swift,Jnosh/swift,OscarSwanros/swift,jtbandes/swift,uasys/swift,jckarter/swift,stephentyrone/swift,djwbrown/swift,alblue/swift,sschiau/swift,sschiau/swift,CodaFi/swift,tardieu/swift,danielmartin/swift,danielmartin/swift,hughbe/swift,jtbandes/swift,jopamer/swift,amraboelela/swift,tjw/swift,xedin/swift,harlanhaskins/swift,tjw/swift,practicalswift/swift,lorentey/swift,swiftix/swift,gmilos/swift,apple/swift,shajrawi/swift,glessard/swift,devincoughlin/swift,ben-ng/swift,practicalswift/swift,shajrawi/swift,OscarSwanros/swift,ahoppen/swift,bitjammer/swift,codestergit/swift,tjw/swift,atrick/swift,parkera/swift,shajrawi/swift,parkera/swift,Jnosh/swift,kperryua/swift,return/swift,harlanhaskins/swift,apple/swift,jckarter/swift,benlangmuir/swift,tinysun212/swift-windows,arvedviehweger/swift,ahoppen/swift,hughbe/swift,jopamer/swift,amraboelela/swift,therealbnut/swift,jmgc/swift,atrick/swift,swiftix/swift,zisko/swift,calebd/swift,sschiau/swift,rudkx/swift,brentdax/swift,brentdax/swift,hooman/swift,devincoughlin/swift,xedin/swift,tardieu/swift,hooman/swift,allevato/swift,shajrawi/swift,return/swift,arvedviehweger/swift,swiftix/swift,austinzheng/swift,natecook1000/swift,jtbandes/swift,frootloops/swift,deyton/swift,codestergit/swift,roambotics/swift,felix91gr/swift,xedin/swift,shahmishal/swift,tkremenek/swift,tardieu/swift,djwbrown/swift,huonw/swift,frootloops/swift,karwa/swift,aschwaighofer/swift,harlanhaskins/swift,JaSpa/swift,calebd/swift,xedin/swift,ben-ng/swift,shahmishal/swift,Jnosh/swift,tjw/swift,stephentyrone/swift,hughbe/swift,lorentey/swift,shahmishal/swift,therealbnut/swift,benlangmuir/swift,huonw/swift,xwu/swift,gribozavr/swift,djwbrown/swift,sschiau/swift,CodaFi/swift,kstaring/swift,airspeedswift/swift,practicalswift/swift,gottesmm/swift,bitjammer/swift,manavgabhawala/swift,kperryua/swift,huonw/swift,allevato/swift,nathawes/swift,karwa/swift,felix91gr/swift,sschiau/swift,milseman/swift,OscarSwanros/swift,arvedviehweger/swift,tjw/swift,ahoppen/swift,jopamer/swift,arvedviehweger/swift,codestergit/swift,glessard/swift,aschwaighofer/swift,therealbnut/swift,practicalswift/swift,zisko/swift,brentdax/swift,parkera/swift,jtbandes/swift,airspeedswift/swift,tkremenek/swift,tinysun212/swift-windows,benlangmuir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