[clang] [CIR] Add cir.lifetime.start and cir.lifetime.end Op (PR #199599)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

TBH, it seems odd that we have a lifetime operation markers, rather than an actual scope.

I believe that our lifetime management should be matching to the 'scopes' that they are in, and better reflect the C/C++ scoping rules?  

SO I guess I would envision more of a `lifetime-scope` thing, that marks the list of allocas on `entry`.

But these markers seem too low level entirely.

https://github.com/llvm/llvm-project/pull/199599