Fix compiler error in CoreFoundation when building in C++ mode (#5081)

`__CFAllocatorRespectsHintZeroWhenAllocating` has two declarations in different headers: `ForFoundationOnly.h` and `ForSwiftFoundationOnly.h`. One of the declarations was under `extern "C"` block, the other one wasn't. Clang accepts this in C language mode, but it is a hard compiler error in C++ language mode due to a linkage mismatch.

This was blocking clients from using CoreFoundation in Swift projects that enable C++ interoperability.

This change makes sure that both declarations of `__CFAllocatorRespectsHintZeroWhenAllocating` are under `extern "C"` blocks.

Sources/CoreFoundation/include/ForFoundationOnly.h

@@ -57,6 +57,8 @@ CF_IMPLICIT_BRIDGING_DISABLED
 #include <mach/mach_time.h>
 #endif
 
+_CF_EXPORT_SCOPE_BEGIN
+
 #if __BLOCKS__
 /* These functions implement standard error handling for reallocation. Their parameters match their unsafe variants (realloc/CFAllocatorReallocate). They differ from reallocf as they provide a chance for you to clean up a buffers contents (in addition to freeing the buffer, etc.)
  
@@ -69,10 +71,13 @@ CF_EXPORT void *_Nonnull __CFSafelyReallocateWithAllocator(CFAllocatorRef _Nulla
 #endif
 
 Boolean __CFAllocatorRespectsHintZeroWhenAllocating(CFAllocatorRef _Nullable allocator);
+
 typedef CF_ENUM(CFOptionFlags, _CFAllocatorHint) {
     _CFAllocatorHintZeroWhenAllocating = 1
 };
 
+_CF_EXPORT_SCOPE_END
+
 #define NSISARGTYPE void * _Nullable
 
 #pragma mark - CFBundle