Rollup of 6 pull requests - #150482

Offload: Build offload as a single Step

r? ``@Kobzol``

Since it looks like we'll postpone enabling offload in CI for a bit, I factored out these improvements which we want independently. I locally tested both build options successfully, the in-tree-clang build, as well as the build where we provide a path to an external clang.

slice iter cleanup: replace checked_sub with saturating_sub

Continuation of rust-lang#146436

r? ``@joboet``

add has_offload/needs-offload to the test infra

unblocks: rust-lang#150426

Mostly copied from rust-lang#131044
lmk if some of these changes should land separately?

r? jieyouxu

fix running stdlib doctests in Miri in CI

r? ``@bjorn3``

Fix enum variant suggestion consuming trailing parenthesis

- Closes rust-lang#150459

Previously the logic incorrectly assumed the variant itself was the function being called when the parent node was a function all. This change adds a check to ensure the replacement span is only expanded if the path expression is actually the callee of the parent expression.

Fix new bors config

I added the root config under `[labels]` by accident, so it failed to be deserialized by bors.