Fix compiler error in CoreFoundation when building in C++ mode

`__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.

Author: egorzhdan

Sources/CoreFoundation/include/ForFoundationOnly.h

@@ -68,7 +68,10 @@ CF_EXPORT void *_Nonnull __CFSafelyReallocate(void * _Nullable destination, size
 CF_EXPORT void *_Nonnull __CFSafelyReallocateWithAllocator(CFAllocatorRef _Nullable, void * _Nullable destination, size_t newCapacity, CFOptionFlags options, void (^_Nullable reallocationFailureHandler)(void *_Nonnull original, bool *_Nonnull outRecovered));
 #endif
 
+_CF_EXPORT_SCOPE_BEGIN
 Boolean __CFAllocatorRespectsHintZeroWhenAllocating(CFAllocatorRef _Nullable allocator);
+_CF_EXPORT_SCOPE_END
+
 typedef CF_ENUM(CFOptionFlags, _CFAllocatorHint) {
     _CFAllocatorHintZeroWhenAllocating = 1
 };