C++ FE: Show both locations in string literal concatenation error

C++ FE: Show both locations in string literal concatenation error

gcc/cp/ChangeLog:
	* parser.c (cp_parser_string_literal): Convert non-standard
	concatenation error to directly use a rich_location, and
	use that to add the location of the first literal to the
	diagnostic.

gcc/testsuite/ChangeLog:
	* g++.dg/diagnostic/string-literal-concat.C: New test case.

From-SVN: r237608