C: Implement musttail attribute for returns

C: Implement musttail attribute for returns

Implement a C23 clang compatible musttail attribute similar to the earlier
C++ implementation in the C parser.

gcc/c/ChangeLog:

	PR c/83324
	* c-parser.cc (struct attr_state): Define with musttail_p.
	(c_parser_statement_after_labels): Handle [[musttail]].
	(c_parser_std_attribute): Dito.
	(c_parser_handle_musttail): Dito.
	(c_parser_compound_statement_nostart): Dito.
	(c_parser_all_labels): Dito.
	(c_parser_statement): Dito.
	* c-tree.h (c_finish_return): Add musttail_p flag.
	* c-typeck.cc (c_finish_return): Handle musttail_p flag.