libstdc++: Better diagnostics for std::format errors

This adds two new static_assert messages to the internals of
std::make_format_args to give better diagnostics for invalid format
args. Rather than just getting an error saying that basic_format_arg
cannot be constructed, we get more specific errors for the cases where
std::formatter isn't specialized for the type at all, and where it's
specialized but only meets the BasicFormatter requirements and so can
only format non-const arguments.

Also add a test for the existing static_assert when constructing a
format_string for non-formattable args.

libstdc++-v3/ChangeLog:

	* include/std/format (_Arg_store::_S_make_elt): Add two
	static_assert checks to give more user-friendly error messages.
	* testsuite/lib/prune.exp (libstdc++-dg-prune): Prune another
	form of "in requirements with" note.
	* testsuite/std/format/arguments/args_neg.cc: Check for
	user-friendly diagnostics for non-formattable types.
	* testsuite/std/format/string_neg.cc: Likewise.

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