Rollup merge of #153726 - Kobzol:parallel-frontend-tests-optional, r=jieyouxu

Add optional CI job to build the compiler with the parallel frontend

Discussed on [Zulip](https://rust-lang.zulipchat.com/#narrow/channel/187679-t-compiler.2Fparallel-rustc/topic/Add.20the.20parallel.20front-end.20test.20suite/with/578684794).

Note: this only builds the compiler, stdlib, etc. with 2 threads. UI tests are still compiled serially. I'd add that in a follow-up PR if we see that this new job works well (and if optional auto jobs work well in the first place).

r? @jieyouxu

Author: JonathanBrouwer

src/tests/ci.md

@@ -109,6 +109,12 @@ The live results can be seen on [the GitHub Actions workflows page].
 At any given time, at most a single `auto` build is being executed.
 Find out more in [Merging PRs serially with bors](#merging-prs-serially-with-bors).
 
+Normally, when an auto job fails, the whole CI workflow immediately ends. However, it can be useful to
+create auto jobs that are "non-blocking", or optional, to test them on CI for some time before blocking
+merges on them. This can be useful if those jobs can be flaky.
+
+To do that, prefix such a job with `optional-`, and set `continue_on_error: true` for it in [`jobs.yml`]. 
+
 [platform tiers]: https://forge.rust-lang.org/release/platform-support.html#rust-platform-support
 [auto]: https://github.com/rust-lang/rust/tree/automation/bors/auto