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

Diff for: src/building/bootstrapping/debugging-bootstrap.md

@@ -76,6 +76,14 @@ $ BOOTSTRAP_TRACING=CONFIG_HANDLING=TRACE ./x build library --stage 1
 
 [tracing-env-filter]: https://docs.rs/tracing-subscriber/0.3.19/tracing_subscriber/filter/struct.EnvFilter.html
 
+##### FIXME(#96176): specific tracing for `compiler()` vs `compiler_for()`
+
+The additional targets `COMPILER` and `COMPILER_FOR` are used to help trace what
+`builder.compiler()` and `builder.compiler_for()` does. They should be removed
+if [#96176][cleanup-compiler-for] is resolved.
+
+[cleanup-compiler-for]: https://github.com/rust-lang/rust/issues/96176
+
 ### Using `tracing` in bootstrap
 
 Both `tracing::*` macros and the `tracing::instrument` proc-macro attribute need to be gated behind `tracing` feature. Examples: