Replicate the APIs of ScalarTargetTransformInfo and
VectorTargetTransformInfo into the TargetTransformInfo pass,
implementing them be delegating back out to the two subobjects.

This is the first step to folding the interfaces together and making
TargetTransformInfo a normal analysis pass (specifically an analysis
group which targets can provide target-specific analysis pass
implementations of).

No callers are migrated here, this just stubs out the interface. Next
step will be to migrate all the callers to directly operate on TTI
instead of STTI or VTTI respectively. That will allow replacing the
machinery for delivering TTI without changing every caller at once.

WIP, I promise all the duplicated interfaces will be removed in the end,
this just decouples the steps of the process.