Don't rebuild LLVM on non-related changes to config.toml

[Mark-Simulacrum]

#42429 closed #35199; but ideally we don't want to rebuild LLVM no matter what; since that has a high overhead for changes that aren't related. See the pull request for more details (#42429 (comment)).

[venkatagiri]

@Mark-Simulacrum Now that rust-lang/cargo#4125 has landed, would below solution work to fix this issue?
In bootstrap, set env var CFG_LLVM_CONFIG to llvm-config path specified in config.toml and in rustc_llvm, print cargo:rerun-if-env-changed=CFG_LLVM_CONFIG?

[Mark-Simulacrum]

Sounds reasonable! I think we'd need to be careful we only read the llvm-config path, but I believe that's true today. Would you like to make a PR?

[venkatagiri]

Yes. I will. Looking at the code again, I feel we can use the LLVM_CONFIG env var (which points to llvm-config based on config.toml or custom llvm build) to trigger llvm rebuild by printing cargo:rerun-if-env-changed=LLVM_CONFIG and removing all changes done in #42429.