Objective-C++ : Fix handling of unnamed message parms [PR49070].

Objective-C++ : Fix handling of unnamed message parms [PR49070].

When we are parsing an Objective-C++ message, a colon is a valid
terminator for a assignment-expression.  That is:

[receiver meth:x:x:x:x];

Is a valid, if somewhat unreadable, construction; corresponding
to a method declaration like:

- (id) meth:(id)arg0 :(id)arg1 :(id)arg2 :(id)arg3;

Where three of the message params have no selector name.

If fact, although it might be unintentional, Objective-C/C++ can
accept message selectors with all the parms unnamed (this applies
to the clang implementation too, which is taken as the reference
for the language).

For regular C++, the pattern x:x is not valid in that position an
an error is emitted with a fixit for the expected scope token.

If we simply made that error conditional on !c_dialect_objc()
that would regress Objective-C++ diagnostics for cases outside a
message selector, so we add a state flag for this.

gcc/cp/ChangeLog:

	PR objc++/49070
	* parser.c (cp_debug_parser): Add Objective-C++ message
	state flag.
	(cp_parser_nested_name_specifier_opt): Allow colon to
	terminate an assignment-expression when parsing Objective-
	C++ messages.
	(cp_parser_objc_message_expression): Set and clear message
	parsing state on entry and exit.
	* parser.h (struct cp_parser): Add a context flag for
	Objective-C++ message state.

gcc/testsuite/ChangeLog:

	PR objc++/49070
	* obj-c++.dg/pr49070.mm: New test.
	* objc.dg/unnamed-parms.m: New test.