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 ```  ``` $ BOOTSTRAP_TRACING=bootstrap=trace,COMPILER=trace,COMPILER_FOR=trace ./x build library ```  r? ``@onur-ozkan`` (or reroll)