[DirectX] Figure out vector alignments and data layout

[llvm-beanz]

A few years ago I posted a patch to phabricator to try and represent DXIL's odd vector alignment rules in the LLVM data layout (https://reviews.llvm.org/D133379).

That patch isn't quite right, but we likely need to figure out something in order to get correct vector alignment especially as we're trying to add vectors to DXIL in SM 6.9.

AC:

• figure out the plan
• likely requires an RFC which'll need iteration etc.

[damyanp]

#124696 is blocked on this, so moving this one to the same epic. We need to decide if the workaround in #160955 is a sufficiently good band-aid to allow us to punt on resolving this issue properly.