Objective-C/C++ : Improve '@' keyword locations.

Objective-C/C++ : Improve '@' keyword locations.

When we are lexing tokens for Objective-C, we combine '@' tokens
with a following keyword (when that keyword is a valid Objective-C
one or, for Objective-C, one of the C++ keywords that can appear in
this position).  The responsibility is passed on to the parser to
validate the resulting combination.

The combination of tokens was being done without applying the rule
to their locations - so that we get:

@property
^

instead of what the user might expect:

@property
^~~~~~~~~

This patch combines the source range of the keyword with that of the
'@' sign - which improves diagnostics.

gcc/c-family/ChangeLog:

	* c-lex.c (c_lex_with_flags): When combining '@' with a
	keyword for Objective-C, combine the location ranges too.