c, objc: Fix up c_parser_objc_selector_arg after CPP_SCOPE changes [PR94637]

Similarly to inline asm, :: (or any other number of consecutive colons) can
appear in ObjC @selector argument and with the introduction of CPP_SCOPE
into the C FE, we need to trat CPP_SCOPE as two CPP_COLON tokens.
The C++ FE does that already that way.

2020-04-19  Jakub Jelinek  <jakub@redhat.com>

	PR objc/94637
	* c-parser.c (c_parser_objc_selector_arg): Handle CPP_SCOPE like
	two CPP_COLON tokens.

	* objc.dg/pr94637.m: New test.