Diagnose const atomics in __atomic builtins.

Diagnose const atomics in __atomic builtins.

Diagnose when a pointer to const T is used as the first argument in at atomic
builtin unless that builtin is a load operation. This is already checked for
C11 atomics builtins but not for __atomic ones.

This patch was given the LGTM by rsmith when it was part
of a larger review. (See http://reviews.llvm.org/D10407)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@249252 91177308-0d34-0410-b5e6-96231b3b80d8