Support for C1x _Atomic specifier (see testcase). This is primarily being...

Support for C1x _Atomic specifier (see testcase).  This is primarily being committed at the moment to help support C++0x <atomic>, but it should be a solid base for implementing the full specification of C1x _Atomic.

Thanks to Jeffrey Yasskin for the thorough review!

llvm-svn: 141330