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)'
```

Author: compnerd

include/swift/AST/AutoDiff.h

@@ -169,7 +169,8 @@ template <> struct DenseMapInfo<AutoDiffDerivativeFunctionKind> {
 
   static bool isEqual(const AutoDiffDerivativeFunctionKind &LHS,
                       const AutoDiffDerivativeFunctionKind &RHS) {
-    return LHS == RHS;
+    return static_cast<AutoDiffDerivativeFunctionKind::innerty>(LHS) ==
+        static_cast<AutoDiffDerivativeFunctionKind::innerty>(RHS);
   }
 };