[AST] Improve support of external layouts in `MicrosoftRecordLayoutBuilder`

Summary:
This patch fixes several small problems with external layouts support in
`MicrosoftRecordLayoutBuilder`:
- aligns properly the size of a struct that ends with a bit field. It was
  aligned on byte before, not on the size of the field, so the struct size was
  smaller than it should be;
- adjusts the struct size when injecting a vbptr in the case when there were no
  bases or fields allocated after the vbptr. Similarly, without the adjustment
  the struct was smaller than it should be;
- the same fix as above for the vfptr.
All these fixes affect the non-virtual size of a struct, so they are tested
through non-virtual inheritance.

Reviewers: rnk, zturner, rsmith

Reviewed By: rnk

Subscribers: jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58544

llvm-svn: 356047