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

[Amy Huang via cfe-commits]

amykhuang wrote:

Added a section for target specific spellings in `GenerateHasAttrSpellingStringSwitch`, and a check for "existsInTarget" in `ParseCXX11AttributeArgs`, and looks like all tests are passing now.

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