[IR] Introduce !captures metadata (#160913)

This introduces `!captures` metadata on stores, which looks like this:

```
store ptr %x, ptr %y, !captures !{!"address", !"read_provenance"}
```

The semantics are the same as replacing the store with a call like this:
```
call void @llvm.store(ptr captures(address, read_provenance) %x, ptr %y)
```

This metadata is intended for annotation by frontends -- it's not
something we can feasibly infer at this point, as it would require
analyzing uses of the pointer stored in memory.

The motivating use case for this is Rust's `println!()` machinery, which
involves storing a reference to the value inside a structure. This means
that printing code (including conditional debugging code), can inhibit
optimizations because the pointer escapes. With the new metadata we can
annotate this as a read-only capture, which has less impact on
optimizations.

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