Add a new `download-ci-llvm = if-unchanged` option and enable it by default for `profile = codegen`

[jyn514]

The codegen profile is intended not just for people working on LLVM itself, but also anyone who touches codegen but not LLVM. For example, anyone who modifies rustc_codegen_ssa or rustc_codegen_llvm is only changing the IR we send to LLVM, not LLVM itself. We should keep llvm.debug-assertions on for them but not for profile = "compiler", but ideally we wouldn't build LLVM from source if it's unmodified.

@majaha points out some things that we should fix before enabling that option by default for the codegen profile:

### Tasks
- [ ] Add a new `if-unchanged` option for `download-ci-llvm`. This should be modeled off the existing option for `download-rustc`, and imply `if-available` (i.e. we should treat this the same as `false` for tier 3 platforms). It may be as simple as extracting out the same `last_modified_commit` function used in https://github.com/rust-lang/rust/blob/8ad89068e2558940b4184d46763ff9c6b325883b/src/bootstrap/min_config.rs#L178-L180 and calling it with `["src/llvm-project"]`, then adding the option in config.toml.
- [ ]  The downloadable llvm for this profile should include all the build artifacts that you get with a source build, so that the build dir is left in a state _as if_ you've just run a full source build. See https://github.com/rust-lang/rust/pull/110074#issuecomment-1500888030 for a full list of artifacts that should be included.
- [ ] The submodule should be downloaded during `./x.py setup` after you select the `codegen` profile.

Originally posted by @jyn514 in #110074 (comment)

[KittyBorgX]

@rustbot claim
Should all 3 tasks be in one PR or is it 3 separate PR's?

[jyn514]

@KittyBorgX I'd say it depends how challenging they are - if it's simple to fix all three in one PR that seems fine, but if it takes a more than a day to do one of the tasks, it probably makes sense to open a PR for it early.

[KittyBorgX]

I think I can tackle points 1 and 3 by tonight so I'll make that into one PR. I'll open a separate PR for the second point, since I think it has more work involved, if that's alright :)

[KittyBorgX]

@rustbot release-assignment