[clang] [CIR] Handle proper hoisting of const variables with InvariantGroupOp (PR #175037)

[Sirui Mu via cfe-commits]

Lancern wrote:

> I find myself thinking that instead of this being a property of a load/store, perhaps this should be a property of the loop or alloca. WDYT? So the following:

A relevant issue we met when originally trying implement this in the incubator is to emit LLVM lifetime markers for local variables. The problems are actually very alike:

- To emit proper LLVM lifetime markers, we need some CIR operation that marks the lifetime span of the alloca, even if it is hoisted.
- If the local variable is `const`, we need to explicitly mark its constness in that same lifetime span.

So you could think of InvariantGroupOp as some sort of "lifetime markers for const local variables", but we don't leverage that for emitting LLVM lifetime markers for now. We could materialize this idea into a general "variable lifetime marker" operation: (the op name is randomly chosen without much thought)

```mlir
cir.func @foo() {
  %x.slot = cir.alloca !s32i
  cir.scope {
    cir.while {
      %x = cir.start_lifetime %x.slot [const]
      // ...
    }
  }
}
```

https://github.com/llvm/llvm-project/pull/175037