[RemoteMirror] Use __attribute__((deprecated)) instead of [[deprecated]].

Use __attribute__((deprecated)) in SwiftRemoteMirror.h as it needs to be includable from C.

Author: mikeash

include/swift/SwiftRemoteMirror/SwiftRemoteMirror.h

@@ -188,7 +188,7 @@ swift_reflection_typeRefForMangledTypeName(SwiftReflectionContextRef ContextRef,
 ///
 /// The returned string is heap allocated and the caller must free() it when
 /// done.
-[[deprecated("Please use swift_reflection_copyNameForTypeRef()")]]
+__attribute__((deprecated("Please use swift_reflection_copyNameForTypeRef()")))
 SWIFT_REMOTE_MIRROR_LINKAGE
 char *
 swift_reflection_copyDemangledNameForTypeRef(