[libcxx] Apply pragma for silencing warning when building with clang-cl too

This silences warnings about unused functions (in an anonymous
namespace).

Differential Revision: https://reviews.llvm.org/D98036

libcxx/src/filesystem/filesystem_common.h

@@ -35,7 +35,7 @@
 #endif
 #endif
 
-#if defined(__GNUC__)
+#if defined(__GNUC__) || defined(__clang__)
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wunused-function"
 #endif