AST: explicitly indicate operator==
AST: explicitly indicate operator==

This is needed to build with VS2019:

```
swift\include\swift\AST\AutoDiff.h(172): error C2593: 'operator ==' is ambiguous
swift\include\swift\Basic\AnyValue.h(129): note: could be 'bool swift::operator ==(const swift::AnyValue &,const swift::AnyValue &)' [found using argument-dependent lookup]
swift\include\swift\AST\AutoDiff.h(172): note: or       'built-in C++ operator==(swift::AutoDiffDerivativeFunctionKind::innerty, swift::AutoDiffDerivativeFunctionKind::innerty)'
swift\include\swift\AST\AutoDiff.h(172): note: while trying to match the argument list '(const swift::AutoDiffDerivativeFunctionKind, const swift::AutoDiffDerivativeFunctionKind)'
```

benlangmuir/swift,CodaFi/swift,parkera/swift,ahoppen/swift,allevato/swift,CodaFi/swift,gribozavr/swift,atrick/swift,gribozavr/swift,stephentyrone/swift,apple/swift,roambotics/swift,glessard/swift,xwu/swift,apple/swift,xwu/swift,ahoppen/swift,benlangmuir/swift,allevato/swift,nathawes/swift,parkera/swift,jmgc/swift,hooman/swift,lorentey/swift,gregomni/swift,airspeedswift/swift,harlanhaskins/swift,nathawes/swift,roambotics/swift,ahoppen/swift,gribozavr/swift,rudkx/swift,gregomni/swift,aschwaighofer/swift,aschwaighofer/swift,airspeedswift/swift,airspeedswift/swift,tkremenek/swift,jckarter/swift,roambotics/swift,airspeedswift/swift,nathawes/swift,atrick/swift,jckarter/swift,karwa/swift,atrick/swift,allevato/swift,ahoppen/swift,atrick/swift,JGiola/swift,xwu/swift,allevato/swift,ahoppen/swift,harlanhaskins/swift,aschwaighofer/swift,gregomni/swift,roambotics/swift,hooman/swift,harlanhaskins/swift,jmgc/swift,roambotics/swift,gribozavr/swift,lorentey/swift,hooman/swift,jckarter/swift,jckarter/swift,glessard/swift,apple/swift,stephentyrone/swift,parkera/swift,xwu/swift,atrick/swift,jmgc/swift,nathawes/swift,karwa/swift,JGiola/swift,apple/swift,airspeedswift/swift,nathawes/swift,lorentey/swift,airspeedswift/swift,lorentey/swift,rudkx/swift,JGiola/swift,parkera/swift,tkremenek/swift,hooman/swift,allevato/swift,karwa/swift,aschwaighofer/swift,gribozavr/swift,harlanhaskins/swift,karwa/swift,tkremenek/swift,jckarter/swift,parkera/swift,apple/swift,glessard/swift,gribozavr/swift,glessard/swift,rudkx/swift,rudkx/swift,allevato/swift,harlanhaskins/swift,JGiola/swift,xwu/swift,karwa/swift,stephentyrone/swift,CodaFi/swift,aschwaighofer/swift,benlangmuir/swift,atrick/swift,hooman/swift,gregomni/swift,stephentyrone/swift,parkera/swift,nathawes/swift,CodaFi/swift,benlangmuir/swift,tkremenek/swift,xwu/swift,tkremenek/swift,rudkx/swift,aschwaighofer/swift,lorentey/swift,benlangmuir/swift,gregomni/swift,gribozavr/swift,jckarter/swift,hooman/swift,stephentyrone/swift,gregomni/swift,CodaFi/swift,stephentyrone/swift,stephentyrone/swift,jmgc/swift,allevato/swift,karwa/swift,parkera/swift,hooman/swift,harlanhaskins/swift,xwu/swift,jckarter/swift,karwa/swift,glessard/swift,ahoppen/swift,lorentey/swift,glessard/swift,roambotics/swift,CodaFi/swift,jmgc/swift,CodaFi/swift,karwa/swift,parkera/swift,rudkx/swift,benlangmuir/swift,airspeedswift/swift,nathawes/swift,lorentey/swift,harlanhaskins/swift,JGiola/swift,gribozavr/swift,apple/swift,tkremenek/swift,JGiola/swift,jmgc/swift,lorentey/swift,tkremenek/swift,aschwaighofer/swift,jmgc/swift