[clang] [Clang] Add fake use emission to Clang with -fextend-lifetimes (PR #110102)

[Jeremy Morse via cfe-commits]

https://github.com/jmorse commented:

Looking good, some comments and questions.

Having to fiddle with `findDominatingStoreToReturnValue` to make it skip fake-uses is slightly worrying, but it also says it's a heuristic and thus probably not fundamental for correctness?

https://github.com/llvm/llvm-project/pull/110102