The check should be done in the semantic phase.

Reviewed-by: Roberto Raggi

This reverts commit 2a59d2ae.

This reverts commit f4163b8b.

Because apparently, while designing the Objective-C language, somebody
thought it was a world-class idea to allow any white-space between the
'@' character and the subsequent keyword. With this fix, we now
correctly parse:
    @  dynamic
and:
    @
      selector
and:
    @"foo"
     "bar"
    @"mooze"
(This last one is 1 single string split over multiple lines.)
Wonderful, isn't it?

What we (and Clang) do not support, but what GCC supports is something
like:
   @"foo"@@  "bar"  @"mooze"  @@
which is equivalent to @"foobarmooze".

When a PP line ended with a \ (join line) and the next line was empty
(ending th joining), the following token would be incorrectly marked as
joined.

Done-with: Roberto Raggi