Improve C++11 attribute parsing.

Improve C++11 attribute parsing.

- General C++11 attributes were previously parsed and ignored. Now they are parsed and stored in AST.
- Add support to parse arguments of attributes that in 'gnu' namespace.
- Differentiate unknown attributes and known attributes that can't be applied to statements when emitting diagnostic.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@165082 91177308-0d34-0410-b5e6-96231b3b80d8