re PR preprocessor/36320 (Required diagnosis of syntax error missed)

re PR preprocessor/36320 (Required diagnosis of syntax error missed)

gcc/testsuite
	PR preprocessor/36320:
	* gcc.dg/cpp/pr36320.c: New file.
libcpp
	PR preprocessor/36320:
	* internal.h (_cpp_parse_expr): Update.
	* expr.c (_cpp_parse_expr): Add 'is_if' argument.  Update error
	messages.
	* directives.c (do_if): Update.
	(do_elif): Require expression if processing group.

From-SVN: r136209