Fix the atomics sema code to convert operands to the argument types

Fix the atomics sema code to convert operands to the argument types
of the underlying _N builtin, not the the type of the pointee of the
actual type.  This ensures that atomics involving pointers end up
using the correct integer type when they are resolved, avoiding
aborts in codegen.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@71218 91177308-0d34-0410-b5e6-96231b3b80d8