[NFC][MSan] Round up OffsetPtr in PoisonMembers

getFieldOffset(layoutStartOffset)  is expected to point to the first trivial
field or the one which follows non-trivial. So it must be byte aligned already.
However this is not obvious without assumptions about callers.
This patch will avoid the need in such assumptions.

Depends on D92727.

Differential Revision: https://reviews.llvm.org/D92728