[llvm] Define __has_warning in Compiler.h if it isn't defined

Follow-up to #141091.

Author: nico

llvm/include/llvm/Support/Compiler.h

@@ -39,6 +39,10 @@
 # define __has_builtin(x) 0
 #endif
 
+#ifndef __has_warning
+# define __has_warning(x) 0
+#endif
+
 // Only use __has_cpp_attribute in C++ mode. GCC defines __has_cpp_attribute in
 // C mode, but the :: in __has_cpp_attribute(scoped::attribute) is invalid.
 #ifndef LLVM_HAS_CPP_ATTRIBUTE