libcpp: Implement P2362R3 - Remove non-encodable wide character literals and multicharacter [PR106647]

My understanding of the paper is that we just want to promote the CPP_WCHAR
"character constant too long for its type" warning to error as it is already
error for u8, u and U literals.

2022-08-26  Jakub Jelinek  <jakub@redhat.com>

	PR c++/106647
	* charset.cc (wide_str_to_charconst): Implement P2362R3 - Remove
	non-encodable wide character literals and multicharacter.  For
	C++23 use CPP_DL_ERROR instead of CPP_DL_WARNING for
	"character constant too long for its type" diagnostics on CPP_WCHAR
	literals.

	* g++.dg/cpp23/wchar-multi1.C: New test.
	* g++.dg/cpp23/wchar-multi2.C: New test.