[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
diff --git a/libcxx/include/algorithm b/libcxx/include/algorithm
index 87e4ec9..c57a98a 100644
--- a/libcxx/include/algorithm
+++ b/libcxx/include/algorithm
@@ -774,7 +774,7 @@
#include <__algorithm/upper_bound.h>
#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#pragma GCC system_header
+# pragma GCC system_header
#endif
#if defined(_LIBCPP_HAS_PARALLEL_ALGORITHMS) && _LIBCPP_STD_VER >= 17
