[libc++][format] Enable format_error on older compilers.

It seems like modifying the header doesn't cause libc++ to be rebuild.
So the breakage of the previous commit didn't happen on my system.

This should fix the build of https://buildkite.com/mlir/mlir-core

Author: mordante

libcxx/include/format

@@ -75,12 +75,6 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER > 17
 
-// TODO FMT Remove this once we require compilers with proper C++20 support.
-// If the compiler has no concepts support, the format header will be disabled.
-// Without concepts support enable_if needs to be used and that too much effort
-// to support compilers with partial C++20 support.
-#if !defined(_LIBCPP_HAS_NO_CONCEPTS) && !defined(_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED)
-
 class _LIBCPP_EXCEPTION_ABI format_error : public runtime_error {
 public:
   _LIBCPP_INLINE_VISIBILITY explicit format_error(const string& __s)
@@ -90,6 +84,12 @@ public:
   virtual ~format_error() noexcept;
 };
 
+// TODO FMT Remove this once we require compilers with proper C++20 support.
+// If the compiler has no concepts support, the format header will be disabled.
+// Without concepts support enable_if needs to be used and that too much effort
+// to support compilers with partial C++20 support.
+#if !defined(_LIBCPP_HAS_NO_CONCEPTS) && !defined(_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED)
+
 _LIBCPP_NORETURN inline _LIBCPP_INLINE_VISIBILITY void
 __throw_format_error(const char* __s) {
 #ifndef _LIBCPP_NO_EXCEPTIONS