[OpenMP] Silence build warnings when built with MinGW

There's an attempt to upstream this change in
https://github.com/intel/ittapi/pull/25 too.

Differential Revision: https://reviews.llvm.org/D114069

openmp/runtime/src/thirdparty/ittnotify/disable_warnings.h

@@ -11,6 +11,8 @@
 
 #if ITT_PLATFORM == ITT_PLATFORM_WIN
 
+#if defined _MSC_VER
+
 #pragma warning(disable : 593) /* parameter "XXXX" was set but never used */
 #pragma warning(disable : 344) /* typedef name has already been declared (with \
                                   same type) */
@@ -19,6 +21,8 @@
 #pragma warning(                                                               \
     disable : 4306) /* conversion from '?' to '?' of greater size */
 
+#endif /* _MSC_VER */
+
 #endif /* ITT_PLATFORM==ITT_PLATFORM_WIN */
 
 #if defined __INTEL_COMPILER