[libcxx] Silence -Wformat-nonliteral warnings in the Windows support code

This brings the mingw build back to zero build warnings as it was
at some earlier time.

Differential Revision: https://reviews.llvm.org/D119134

(cherry picked from commit 8cffea0)

Author: mstorsjo

libcxx/src/support/win32/locale_win32.cpp

@@ -97,7 +97,10 @@ int snprintf_l(char *ret, size_t n, locale_t loc, const char *format, ...)
         ret, n, format, loc, ap);
 #else
     __libcpp_locale_guard __current(loc);
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wformat-nonliteral"
     int result = vsnprintf( ret, n, format, ap );
+#pragma clang diagnostic pop
 #endif
     va_end(ap);
     return result;

libcxx/src/support/win32/support.cpp

@@ -23,7 +23,10 @@ int __libcpp_vasprintf( char **sptr, const char *__restrict format, va_list ap )
     // Query the count required.
     va_list ap_copy;
     va_copy(ap_copy, ap);
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wformat-nonliteral"
     int count = vsnprintf( NULL, 0, format, ap_copy );
+#pragma clang diagnostic pop
     va_end(ap_copy);
     if (count < 0)
         return count;
@@ -33,7 +36,10 @@ int __libcpp_vasprintf( char **sptr, const char *__restrict format, va_list ap )
         return -1;
     // If we haven't used exactly what was required, something is wrong.
     // Maybe bug in vsnprintf. Report the error and return.
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wformat-nonliteral"
     if (vsnprintf(p, buffer_size, format, ap) != count) {
+#pragma clang diagnostic pop
         free(p);
         return -1;
     }