Factor out static_assert constexpr string extraction for reuse

Factor out static_assert constexpr string extraction for reuse

The only semantics changes are slightly more vague error messages
to generalize.

gcc/cp/ChangeLog:

	* cp-tree.h (class cexpr_str): Add.
	* semantics.cc (finish_static_assert): Convert to use cexpr_str.
	(cexpr_str::type_check): Extract constexpr string code to here.
	(cexpr_str::extract): ... and here.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp26/static_assert1.C: Update to new error message.
	* g++.dg/cpp0x/udlit-error1.C: Dito.