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)

llvm-svn: 249252