Support _Atomic and _Atomic ( type ) about language-c

[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.