[clang] [Clang] Add "extend lifetime" flags and release note (PR #110000)

[Jeremy Morse via cfe-commits]

jmorse wrote:

The additions here look fine; however I think there's generally precedent that anything we add needs to have /some/ kind of test, or be covered by something already existing, otherwise we're vulnerable to:
 * Patch lands,
 * Someone refactors clang switch handling,
 * Other patches land but the flag doesn't do anything / is rejected,
 * Confusion reigns

As far as I understand it, these are driver options that will be passed through to cc1? In that case we can at least test the passthrough, i.e. that `-fextend-lifetimes` appears in the output of `-###`.

https://github.com/llvm/llvm-project/pull/110000