[NFC] Check for defined(__GNUC__) before use

This silences some spurious warnings on Windows builds that __GNUC__ is
not defined. These warnings make Windows builds very noisy.

Author: DavidTruby

llvm/include/llvm/ADT/iterator_range.h

@@ -43,7 +43,8 @@ class iterator_range {
   IteratorT begin_iterator, end_iterator;
 
 public:
-#if __GNUC__ == 7 || (__GNUC__ == 8 && __GNUC_MINOR__ < 4)
+#if defined(__GNUC__) &&                                                       \
+    (__GNUC__ == 7 || (__GNUC__ == 8 && __GNUC_MINOR__ < 4))
   // Be careful no to break gcc-7 and gcc-8 < 8.4 on the mlir target.
   // See https://github.com/llvm/llvm-project/issues/63843
   template <typename Container>