C: fix uninitialized ranges for __alignof__

C: fix uninitialized ranges for __alignof__

gcc/c/ChangeLog:
	* c-parser.c (c_parser_alignof_expression): Capture location of
	closing parenthesis (if any), or of end of unary expression, and
	use it to build a src_range for the expression.

gcc/testsuite/ChangeLog:
	* gcc.dg/plugin/diagnostic-test-expressions-1.c (test_alignof):
	New test function.

From-SVN: r231415