[clang] Recommit "Implement [[msvc::no_unique_address]] (#65675)" (PR #67199)

[Amy Huang via cfe-commits]

amykhuang wrote:

hmm this passes most of the checks in SemaCXX/cxx2a-no-unique-address.cpp, but fails "attribute 'no_unique_address' cannot have an argument list". Also fails Preprocessor/has_attribute.cpp because both attributes are recognized on itanium and windows targets.

https://github.com/llvm/llvm-project/pull/67199