libstdc++: Adjust std::from_chars negative tests

libstdc++: Adjust std::from_chars negative tests

Also test with an enumeration type. Move the dg-error directives outside
the #if block, because DejaGnu would process them whether or not wchar_t
support is present.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/from_chars/1_c++20_neg.cc: Check enumeration
	type.
	* testsuite/20_util/from_chars/1_neg.cc: Likewise. Move dg-error
	directives outside preprocessor condition.