Auto merge of #82653 - jyn514:submodules-on-demand, r=Mark-Simulacrum

Update all submodules that rustbuild doesn't depend on lazily

This only updates the submodules the first time they're needed, instead
of unconditionally the first time you run x.py.

Ideally, this would move *all* submodules to rustbuild and not exclude some tools and
backtrace. Unfortunately, cargo requires all `Cargo.toml` files in the
whole workspace to be present to build any crate.

On my machine, this takes the time for an initial submodule clone (for
`x.py --help`) from 55.70 to 15.87 seconds.

Helps with https://github.com/rust-lang/rust/issues/76653. Builds on https://github.com/rust-lang/rust/pull/86015 and should not be merged before (only the last commit is relevant).

0 files changed, 0 insertions, 0 deletions