download-ci-llvm depends on the repo clone depth

Summary

When using llvm.download-ci-llvm (the default), the hash that gets downloaded depends on the clone depth of the rust repo. This can cause confusing behavior because the LLVM_INVALIDATION_PATHS do not entirely capture what can change LLVM, and different hashes can have different behavior.

Command used

Working on aarch64 macOS. Current main is bb779a91568ac1ee0b8a9dcb6b69219ef30b18a3.

> git clone https://github.com/rust-lang/rust.git
> cd rust
> ./x build library
....
Downloads llvm-1ed488274bec5bf5cfe6bf7a1cc089abcc4ebd68, last modified from LLVM_INVALIDATION_PATHS.
....
  --- stderr
  llvm-config: error: libLLVM-22-rust-1.95.0-nightly.dylib is missing

(This failure is related to https://github.com/rust-lang/rust/issues/153077, which is now fixed, but this is still picking up an old commit.)

However, doing a clone of depth 1 has different behavior:

> git clone --depth=1 https://github.com/rust-lang/rust.git
> cd rust
> ./x build library
....
Downloads llvm-bb779a91568ac1ee0b8a9dcb6b69219ef30b18a3, the current main commit.
....
Build completed successfully in 0:04:30

Expected behaviour

I don't know. Preferably always "just works", but maybe some warnings or something to at least provide some hints that a shallow clone works differently? That would probably be a lot of false positives, though.

Perhaps also LLVM_INVALIDATION_PATHS could include more things like the CI configuration, since those change the resulting LLVM build.

Actual behaviour

Build mysteriously fails and requires a lot of investigation.

Bootstrap configuration (bootstrap.toml)

# empty

Operating system

aarch64 macOS

HEAD

bb779a91568ac1ee0b8a9dcb6b69219ef30b18a3

Start with the llvm.download-ci-llvm path used by ./x build library and inspect how clone depth selects the LLVM hash. Read the handling of LLVM_INVALIDATION_PATHS and the bootstrap configuration, then reproduce the difference between a full and depth-1 clone on aarch64 macOS. Done means the behavior is defined and the confusing failure or warning is addressed.