[MS Compat] Allow _Atomic(Type) and 'struct _Atomic' to coexist

MSVC 2013 ships, as part of its STL implementation, a class named
'_Atomic'.  This is unfortunate because this keyword is in conflict with
the C11 keyword with the same name.  Our solution was to disable this
keyword when targeting MSVC 2013 and reenable it for 2015.

However, this makes it impossible for clang's headers to make use of
_Atomic.  This is problematic in the case of libc++ as it makes heavy
use of this keyword.

Let the keywordness of _Atomic float under certain circumstances:
the body of a class named _Atomic, or a class with a base specifier
named _Atomic, will not have the keyword variant of _Atomic for the
duration of the class body.  This is sufficient to allow us to correctly
handle _Atomic in the STL while permitting us to use _Atomic as a
keyword everywhere else.

Differential Revision: http://reviews.llvm.org/D11233

llvm-svn: 242970