Thin out rustc_codegen_llvm dependencies.

[eddyb]

I'm working on rustc_mir and rustc_codegen_llvm gets rebuilt even though it barely uses anything.
cc @oli-obk @michaelwoerister Can we do this long-term given #46882 and any other plans?

[oli-obk]

should be possible...

things left to do after #46882

• rustc_mir::interpret::{read_target_uint, const_val_field};
• rustc_mir::monomorphize
  • Instance
  • resolve_drop_in_place
  • resolve_closure
  • item::*

[mati865]

Is it small enough?

rust/src/librustc_codegen_llvm/Cargo.toml

Lines 13 to 18 in 38cd948

	[dependencies]
	cc = "1.0.1" # Used to locate MSVC
	num_cpus = "1.0"
	rustc-demangle = "0.1.15"
	rustc_llvm = { path = "../librustc_llvm" }
	memmap = "0.6"

[Mark-Simulacrum]

Cargo.toml is not the source of truth anymore -- sysroot dependencies will become relevant after #61727, and those aren't listed in any one place. A number of crates are currently being pulled from the sysroot.

[eddyb]

This has been solved by #61062. #61727 will have more accurate tracking, but the dependencies are gone.