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

[Richard Smith via cfe-commits]

zygoloid wrote:

Looks like the tests in #110102 are named `extend-liveness-*.cpp`. Perhaps `-fextend-liveness` would be a good name for this flag?

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