Fix -Wunknown-pragmas warnings with gcc in swift demangling code

No intended behavior change.

Author: nico

include/swift/Demangling/ManglingMacros.h

@@ -17,12 +17,12 @@
 #define MANGLE_AS_STRING(M) STRINGIFY_MANGLING(M)
 
 /// The mangling prefix for the new mangling.
-#if !defined(_MSC_VER) || _MSC_VER-0 >= 1926
+#if defined(__clang__)
 _Pragma("clang diagnostic push")
 _Pragma("clang diagnostic ignored \"-Wdollar-in-identifier-extension\"")
 #endif
 #define MANGLING_PREFIX $s
-#if !defined(_MSC_VER) || _MSC_VER-0 >= 1926
+#if defined(__clang__)
 _Pragma("clang diagnostic pop")
 #endif