Different way of checking for support of local warning suppression

Closes #2474

src/catch2/internal/catch_compiler_capabilities.hpp

@@ -63,11 +63,17 @@
 #    define CATCH_INTERNAL_SUPPRESS_UNUSED_VARIABLE_WARNINGS _Pragma( "nv_diag_suppress 177" )
 #endif
 
+// clang-cl defines _MSC_VER as well as __clang__, which could cause the
+// start/stop internal suppression macros to be double defined.
 #if defined(__clang__) && !defined(_MSC_VER)
 
 #    define CATCH_INTERNAL_START_WARNINGS_SUPPRESSION _Pragma( "clang diagnostic push" )
 #    define CATCH_INTERNAL_STOP_WARNINGS_SUPPRESSION  _Pragma( "clang diagnostic pop" )
 
+#endif // __clang__ && !_MSC_VER
+
+#if defined(__clang__)
+
 // As of this writing, IBM XL's implementation of __builtin_constant_p has a bug
 // which results in calls to destructors being emitted for each temporary,
 // without a matching initialization. In practice, this can result in something