[clang] Implement [[msvc::no_unique_address]] (PR #65675)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

When you say you want a second opinion on CodeGen changes, do you mean RecordLayoutBuilder.cpp?  I don't see any non-whitespace changes to clang/lib/CodeGen/ .

https://github.com/llvm/llvm-project/pull/65675