Evaluate compound literals directly into the result aggregate
when that aggregate isn't potentially aliased.
Add CharUnits::alignmentAtOffset.
Change hasAggregateLLVMType, which conflates complex and
aggregate types in a profoundly wrong way that has to be
worked around in every call site, to getEvaluationKind,
which classifies and distinguishes between all of these
cases.

Also, normalize the API for loading and storing complexes.

I'm working on a larger patch and wanted to pull these
changes out, but it would have be annoying to detangle
them from each other.
Move the atomics code into its own file.
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.