[SourceKit] Verify the stdlib is loaded before creating an AST
[SourceKit] Verify the stdlib is loaded before creating an AST

We have seen a couple of crashes that look like they are being caused by `sourcekitd` being unable to load the Swift stdlib. Emit a proper error message explaining the issue instead of crashing.

This change assumes that all sourcekitd requests that create an AST require the stdlib. IMHO this is a reasonable assumption.

Fixes rdar://75740572 by catching the crash in the new check.

atrick/swift,JGiola/swift,atrick/swift,apple/swift,glessard/swift,tkremenek/swift,JGiola/swift,tkremenek/swift,apple/swift,JGiola/swift,glessard/swift,JGiola/swift,parkera/swift,atrick/swift,xwu/swift,tkremenek/swift,rudkx/swift,roambotics/swift,xwu/swift,rudkx/swift,xwu/swift,benlangmuir/swift,atrick/swift,roambotics/swift,apple/swift,hooman/swift,ahoppen/swift,benlangmuir/swift,xwu/swift,parkera/swift,tkremenek/swift,glessard/swift,gregomni/swift,gregomni/swift,xwu/swift,apple/swift,parkera/swift,apple/swift,glessard/swift,benlangmuir/swift,tkremenek/swift,roambotics/swift,gregomni/swift,benlangmuir/swift,atrick/swift,parkera/swift,gregomni/swift,rudkx/swift,benlangmuir/swift,benlangmuir/swift,gregomni/swift,parkera/swift,hooman/swift,parkera/swift,hooman/swift,ahoppen/swift,apple/swift,rudkx/swift,xwu/swift,gregomni/swift,rudkx/swift,JGiola/swift,xwu/swift,hooman/swift,hooman/swift,roambotics/swift,ahoppen/swift,ahoppen/swift,parkera/swift,ahoppen/swift,atrick/swift,tkremenek/swift,hooman/swift,hooman/swift,rudkx/swift,parkera/swift,glessard/swift,ahoppen/swift,roambotics/swift,tkremenek/swift,roambotics/swift,JGiola/swift,glessard/swift