[llvm] [ExtendLifetimes] Implement llvm.fake.use to extend variable lifetimes (PR #86149)

[Stephen Tozer via llvm-commits]

SLTozer wrote:

> I am investigating further to confirm.

Some of the tests added in this patch didn't have an explicit triple, which was necessary for the test to pass - I've already committed updates to add those triples, by the looks of it the test has started passing as of that commit:
https://luci-milo.appspot.com/ui/p/fuchsia/builders/toolchain.ci/clang-base-mac-x64/b8738234501894039297/overview

https://github.com/llvm/llvm-project/pull/86149