Swift-to-C++: `getTypeMetadata` inline helpers that refer to C++ type metadata are annotated as `used` for debug compilations, causing linker errors

[hyp]

as reported on https://forums.swift.org/t/undeclared-use-of-identifier-in-generated-module-swift-file/67894.
It shows up as a linker error like this:

ld: Undefined symbols:
  type metadata accessor for __C.HelloCxxWorld, referenced from:
      swift::TypeMetadataTrait<HelloCxxWorld>::getTypeMetadata() in HelloHello.o

This is caused by how the C++ getTypeMetadata inline function is annotated in the generated header. It uses the SWIFT_INLINE_THUNK annotation which in debug mode in Xcode (when DEBUG is defined as a preprocessor macro/Swift build condition) is forced to be emitted into the object file, even when it's not used. This is wrong, as it's not supposed to be linked in forcefully and should only be needed when it's actually used.

Such functions should be annotated with SWIFT_INLINE_PRIVATE_HELPER instead.