[libc++] Normalize all our '#pragma GCC system_header', and regression-test.
Now we'll notice if a header forgets to include this magic phrase.
Differential Revision: https://reviews.llvm.org/D118800
NOKEYCHECK=True
GitOrigin-RevId: fa6b9e401085c0a210daf81ae79af6dd0144f129
diff --git a/include/filesystem b/include/filesystem
index 74f5965..afb75df 100644
--- a/include/filesystem
+++ b/include/filesystem
@@ -263,7 +263,7 @@
#endif
#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#pragma GCC system_header
+# pragma GCC system_header
#endif
#endif // _LIBCPP_FILESYSTEM