[clang] ✨ [Sema, Lex, Parse] Preprocessor embed in C and C++ (and Obj-C and Obj-C++ by-proxy) (PR #68620)

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h-vetinari wrote:

@AaronBallman, hope the WG14 meeting went well? I've essentially done what I planned to do here, so I was wondering if you're ready to take it from here? I can try to incorporate the review feedback myself, but it's quite likely I'd make a mess of it (not familiar with the clang code base at all). 😅 

https://github.com/llvm/llvm-project/pull/68620