…acrum

Ensure that all LLVM components requested by tests are available on CI

Addresses rust-lang#75064 (comment)

I used an environment variable because passing a command line option all the way from CI to compiletest would be just too much hassle for this task.
I added a new variable, but any of the already existing ones defined by CI could be used instead.
r? @Mark-Simulacrum

…as-schievink

Rollup of 12 pull requests

Successful merges:

 - rust-lang#77037 (more tiny clippy cleanups)
 - rust-lang#77233 (BTreeMap: keep an eye out on the size of the main components)
 - rust-lang#77280 (Ensure that all LLVM components requested by tests are available on CI)
 - rust-lang#77284 (library: Forward compiler-builtins "mem" feature)
 - rust-lang#77296 (liveness: Use Option::None to represent absent live nodes)
 - rust-lang#77322 (Add unstable book docs for `-Zunsound-mir-opts`)
 - rust-lang#77328 (Use `rtassert!` instead of `assert!` from the child process after fork() in std::sys::unix::process::Command::spawn())
 - rust-lang#77331 (Add test for async/await combined with const-generics.)
 - rust-lang#77338 (Fix typo in alloc vec comment)
 - rust-lang#77340 (Alloc vec use imported path)
 - rust-lang#77345 (Add test for issue rust-lang#74761)
 - rust-lang#77348 (Update books)

Failed merges:

r? `@ghost`

This check was introduced by rust-lang#77280 to ensure that all tests that are
filtered by LLVM component are actually tested in CI. However this
causes issues for new targets (e.g. rust-lang#101069) where support is only
available on the latest LLVM version.

This PR restricts the tests to only CI jobs that use the latest LLVM
version.

Allow older LLVM versions to have missing components

This check was introduced by #77280 to ensure that all tests that are filtered by LLVM component are actually tested in CI. However this causes issues for new targets (e.g. #101069) where support is only available on the latest LLVM version.

This PR restricts the tests to only CI jobs that use the latest LLVM version.