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

Description

[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

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