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

[Paul T Robinson via cfe-commits]

https://github.com/pogo59 commented:

It seems a little odd to have only positive forms of these flags; usually toggles would have both positive and negative forms. Maybe @MaskRay has an opinion?

The release note doesn't say: Does `-fextend-lifetimes` imply `-fextend-this-pointer`? They're implemented as independent toggles but the effect isn't really independent IIRC. I wonder (years after it was originally implemented downstream, I know) whether we'd be better off with `-fextend-lifetimes[={all,this,none}]`. (But maybe wait for a second opinion.)

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