testsuite, Objective-C : Fix two plugin diagnostics tests for Darwin.

testsuite, Objective-C : Fix two plugin diagnostics tests for Darwin.

The @selector and @protocol keywords produce a var decl without
useful location information. The current diagnostics plugin does not
look at VAR_DECLs (and it would not be helpful if it did in this
case, since a single decl is shared across all the users).

However, ee can, in this case, make expressions that produce useful
information.

gcc/testsuite/ChangeLog:

	* objc.dg/plugin/diagnostic-test-expressions-1.m: Use assignment
	expressions for @selector and @protocol to obtain a useful
	diagnostic range.