Rollup merge of #137080 - jieyouxu:more-tracing, r=onur-ozkan

bootstrap: add more tracing to compiler/std/llvm flows

- Add more tracing to compiler/std/llvm flows.
- Two drive-by nits:
    1. Take `TargetSelection` by-value for `builder.is_builder_target()`. Noticed while adding tracing; follow-up to #136767.
    2. Coalesce enzyme build logic into one branch.
- Document `COMPILER{,_FOR}` tracing targets for #96176.
- No functional changes.

### Testing

You can play with the tracing locally with:

```
$ BOOTSTRAP_TRACING=bootstrap=debug ./x build library
$ BOOTSTRAP_TRACING=bootstrap=trace ./x build library
$ BOOTSTRAP_TRACING=bootstrap=trace,COMPILER=trace,COMPILER_FOR=trace ./x build library
```

### Previews

```
$ BOOTSTRAP_TRACING=bootstrap=debug ./x build library
```

![Screenshot 2025-02-15 230824](https://github.com/user-attachments/assets/c3b02b62-d52e-4c03-a00a-da0d95618989)

```
$ BOOTSTRAP_TRACING=bootstrap=trace,COMPILER=trace,COMPILER_FOR=trace ./x build library
```

![Screenshot 2025-02-15 233859](https://github.com/user-attachments/assets/842e4ece-4c26-4191-acbb-5f93e42de4dc)

r? ``@onur-ozkan`` (or reroll)

Author: matthiaskrgr