c: Suppress -Wdeprecated-non-prototype warnings for builtins

Builtins defined with BT_FN_INT_VAR etc. show as functions without
a prototype and trigger the warning.

gcc/c/

	PR c/119950
	* c-typeck.cc (convert_arguments): Check for built-in
	function declaration before warning.

gcc/testsuite/

	* gcc.dg/Wdeprecated-non-prototype-5.c: New test.

(cherry picked from commit b6d37ec1dd2a228d94e7b5b438f3aa53684316bc)