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

[Jeremy Morse via cfe-commits]

https://github.com/jmorse commented:

Behold, I've added a bunch of pedantic comments about the tests.

I think a significant matter is that they all run an LLVM optimisation pipeline, which I believe means they cover too much of the project to be "clang" tests, they're more end-to-end or cross project tests. Potentially we can move them there; it's more preferable to test for the exact output of clang without the optimisation passes instead though.

(If there's optimisation behaviour that does need testing, it should be part of the relevant LLVM pass).

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