Auto merge of #81601 - jyn514:llvm-on-demand, r=Mark-Simulacrum

Move llvm submodule updates to rustbuild

This enables better caching, since LLVM is only updated when needed, not
whenever x.py is run. Before, bootstrap.py had to use heuristics to
guess if LLVM would be needed, and updated the module more often than
necessary as a result.

This syncs the LLVM submodule only just before building the compiler, so
people working on the standard library never have to worry about it.
Example output:

```
Copying stage0 std from stage0 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / x86_64-unknown-linux-gnu)
Updating submodule src/llvm-project
Submodule 'src/llvm-project' (https://github.com/rust-lang/llvm-project.git) registered for path 'src/llvm-project'
Submodule path 'src/llvm-project': checked out 'f9a8d70b6e0365ac2172ca6b7f1de0341297458d'
```

Implements https://github.com/rust-lang/rust/issues/76653#issuecomment-770265169. This could be easily extended to other submodules, like `rust-by-example` and `rustc-dev-guide`, which aren't needed for cargo's workspace resolution.

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