Fix macOS build after r358716

COPYFILE_CLONE is only defined on newer macOS versions, using it without
check breaks build on systems running legacy OS and toolchain.

Differential Revision: https://reviews.llvm.org/D65317

llvm-svn: 367084
diff --git a/llvm/lib/Support/Unix/Path.inc b/llvm/lib/Support/Unix/Path.inc
index 908f43e..5c2c061 100644
--- a/llvm/lib/Support/Unix/Path.inc
+++ b/llvm/lib/Support/Unix/Path.inc
@@ -1203,7 +1203,7 @@
 /// implementation.
 std::error_code copy_file(const Twine &From, const Twine &To) {
   uint32_t Flag = COPYFILE_DATA;
-#if __has_builtin(__builtin_available)
+#if __has_builtin(__builtin_available) && defined(COPYFILE_CLONE)
   if (__builtin_available(macos 10.12, *)) {
     bool IsSymlink;
     if (std::error_code Error = is_symlink_file(From, IsSymlink))