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

[Stephen Tozer via cfe-commits]

SLTozer wrote:

I've added a test for this patch that tests the driver - since we will now be relying on the driver to decide whether to pass these flags to the frontend, rather than trivially always doing so (so that [later on](https://github.com/llvm/llvm-project/pull/118026) we can use the driver to enable this at `-Og`), having a test seems useful. Also updated the help text to give a little more detail on usage and explain some key limitations.

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