Make //@ skip-filecheck a normal compiletest directive

[Zalathar]

The skip-filecheck directive is currently used by mir-opt tests, to suppress the default behaviour of running LLVM's FileCheck tool to check MIR output against FileCheck rules in the test file.

The skip-filecheck directive was not included in the big migration to //@ directive syntax (#121370), perhaps because it was parsed and processed in the miropt-test-tools helper crate, not in compiletest itself.

Recently I noticed that a small number of codegen-llvm tests were using the //@ build-pass directive, which has the non-obvious effect of skipping FileCheck in codegen tests. That's quite confusing, so I decided to have the mir-opt tests migrate over to a proper //@ skip-filecheck directive, which could then be used by codegen tests as well.

(I also added skip-filecheck support to assembly tests, which are very similar to codegen tests, though there are currently no assembly tests that actually use //@ skip-filecheck.)

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Support for using //@ build-pass in codegen tests to skip FileCheck was introduced in #113603. With hindsight, I think doing things that way was pretty clearly a mistake, and we'll be better off with //@ skip-filecheck.

r? jieyouxu

[rustbot]

The rustc-dev-guide subtree was changed. If this PR only touches the dev guide consider submitting a PR directly to rust-lang/rustc-dev-guide otherwise thank you for updating the dev guide with your changes.

cc @BoxyUwU, @tshepang

Some changes occurred in src/tools/compiletest

cc @jieyouxu

compiletest directives have been modified. Please add or update docs for the
new or modified directive in src/doc/rustc-dev-guide/.

[jieyouxu]

Oh nice yes, this is something that was mildly annoying to me 👍 I'll take a look soon

[Zalathar]

Non-blocking cc @davidtwco on the fact that tests/codegen-llvm/scalable-vectors/tuple-intrinsics.rs was actually skipping FileCheck, due to its use of //@ build-pass, despite containing several FileCheck assertions.

In that test, skipping FileCheck seems to have been unintentional.

Unfortunately we can't just re-enable FileCheck for that test, because the current FileCheck assertions appear to be broken.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.