[StructLayout] Use TrailingObjects to allocate space for MemberOffsets.

[StructLayout] Use TrailingObjects to allocate space for MemberOffsets.

MemberOffsets are stored at the end of StructLayout. The class
contains a single entry array to mark the start of the member
offsets. getStructLayout calculates the additional space needed
for additional elements before allocating memory.

This patch converts this to use TrailingObjects. This simplifies
the size computation in getStructLayout and gets rid of the
single entry array.

This is prep work, but to use TypeSize instead of uint64_t for
D98169. The single entry array doesn't work with TypeSize because
TypeSize doesn't have a default constructor. We thought this
change was an improvement by itself so we've separated it out.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D99608