Promote atomic type sizes up to a power of two, capped by

Promote atomic type sizes up to a power of two, capped by
MaxAtomicPromoteWidth.  Fix a ton of terrible bugs with
_Atomic types and (non-intrinsic-mediated) loads and stores
thereto.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176658 91177308-0d34-0410-b5e6-96231b3b80d8