[NFC] Check for defined(__GNUC__) before use (llvm#116076)

This silences some spurious warnings on Windows builds with clang-cl
that `__GNUC__` is not defined if `-Wundef` is passed, which is the default 
in MLIR. 
These warnings make Windows builds of LLVM very noisy when MLIR is
included.

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>