C FE: improvements to ranges of bad return values

C FE: improvements to ranges of bad return values

gcc/c/ChangeLog:
	* c-parser.c (c_parser_statement_after_labels): When calling
	c_finish_return, Use the return expression's location if it has
	one, falling back to the location of the first token within it.
	* c-typeck.c (c_finish_return): When issuing warnings about
	the incorrect presence/absence of a return value, issue a note
	showing the declaration of the function.

gcc/testsuite/ChangeLog:
	* gcc.dg/diagnostic-range-bad-return.c: New test case.

From-SVN: r231786