[6.2][cxx-interop] Types exposed from ObjC modules should be behind a macro

Explanation: We the generated reverse interop headers to be valid C++,
so every declaration coming from an Obj-C module should be behind an
ifdef. Unfortunately, we do not always have this information but we do
know that our frameworks contain Obj-C code. So this PR makes sure every
entity coming from our frameworks are behind ifdef.
Issues: rdar://152836730
Original PRs: #83002
Risk: Low, the change is narrow and straightforward.
Testing: Added a compiler test.
Reviewers: @egorzhdan

include/swift/AST/SwiftNameTranslation.h

@@ -112,6 +112,9 @@ inline bool isExposableToCxx(
   return !getDeclRepresentation(VD, isZeroSized).isUnsupported();
 }
 
+bool isObjCxxOnly(const ValueDecl *VD);
+bool isObjCxxOnly(const clang::Decl *D);
+
 /// Returns true if the given value decl D is visible to C++ of its
 /// own accord (i.e. without considering its context)
 bool isVisibleToCxx(const ValueDecl *VD, AccessLevel minRequiredAccess,