[clang] [llvm] [CaptureTracking][FunctionAttrs] Add support for CaptureInfo (PR #125880)

[Nikita Popov via cfe-commits]

nikic wrote:

Thanks for the report! I've reverted the change for now.

```llvm
declare ptr @passthrough(ptr)

define i16 @test() { 
  %a = alloca i16, align 1
  store i16 1, ptr %a, align 1
  %call = call ptr @passthrough(ptr readnone captures(ret: address, provenance) %a)
  %v = load i16, ptr %call, align 1
  ret i16 %v 
}
```

We correctly determine that `%a` is not captured, but because of that we miss the indirect access via the load.

I believe the reason is that `isEscapeSource()` will report that this is an escape source, and thus cannot alias with a non-escaped pointer. However, if `captures(ret: address, provenance)` is used, this is not correct. isEscapeSource() already takes isIntrinsicReturningPointerAliasingArgumentWithoutCapturing() into account for essentially the same reason.

https://github.com/llvm/llvm-project/pull/125880