Support _Atomic and _Atomic ( type )

[visq]

C11 has introduced a rather unpleasant complication.
The keyword _Atomic is allowed to be used in two different contexts:
(1) as a type qualifier if it is not followed by a opening parenthesis
(2) as part of a type specifier if it is followed by ( type )
It is tricky to extend the happy spec to support both forms in a conflict-free way.

[visq]

One solution for LR parsers is implemented in https://github.com/jhjourdan/C11parser/blob/master/lexer.mll. They combine the tokens ATOMIC and LPAREN to ATOMIC_LPAREN, which allows to distinguish the type qualifier ATOMIC and the type specifier ATOMIC_LPAREN type RPAREN.
If the performance of the parser does not drop significantly, this seems like an acceptable solution.

[visq]

Implemented a simple solution that directly recognizes "_Atomic\s*(" as a special token, following the approach described in the previous post.