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

[Erich Keane via cfe-commits]

erichkeane wrote:

> > I'm unfamiliar with `ucf`? Is that a dialect that google doesn't know about? If so, why doesn't said dialect have the lifetime markers rather than CIR?
> 
> Unstructured control flow (blocks + branches).

Ah, thanks :)  Then yes, we'd definitely need that.  Do we/do we intend to do that at the CIR level, or as a part of lowering to other dialects? 

I'm not against these if we intend to do that conversion entirely within CIR, but as @Lancern mentioned above: I want to make sure we're not going to have Clang generate these

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