[Support] On AIX, Check ENOTSUP on posix_fallocate instead of EOPNOTSUPP

Summary:
`posix_fallocate` can fail if the underlying filesystem does not support
it; and, on AIX, such a failure is reported by a return value of
`ENOTSUP`. The existing code checks only for `EOPNOTSUPP`, which may
share the same value as `ENOTSUP`, but is not required to.

Reviewers: xingxue, sfertile, jasonliu

Reviewed By: xingxue

Subscribers: kristina, jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60175

llvm-svn: 357662
diff --git a/llvm/lib/Support/Unix/Path.inc b/llvm/lib/Support/Unix/Path.inc
index a02585c..05ccc6c 100644
--- a/llvm/lib/Support/Unix/Path.inc
+++ b/llvm/lib/Support/Unix/Path.inc
@@ -492,7 +492,12 @@
   // If we have posix_fallocate use it. Unlike ftruncate it always allocates
   // space, so we get an error if the disk is full.
   if (int Err = ::posix_fallocate(FD, 0, Size)) {
-    if (Err != EINVAL && Err != EOPNOTSUPP)
+#ifdef _AIX
+    constexpr int NotSupportedError = ENOTSUP;
+#else
+    constexpr int NotSupportedError = EOPNOTSUPP;
+#endif
+    if (Err != EINVAL && Err != NotSupportedError)
       return std::error_code(Err, std::generic_category());
   }
 #endif