ci: add MSan Linux job

[mizvekov]

Adds MSan sanitizer job.

This builds libc++, if the host is capable of it and for non-release jobs, and uses it instead of the system library.

This libc++ will be built with whatever sanitizer is selected, increasing the effectiveness of those tests.
This is also a requirement for MSan, which requires everything to be compiled with it.

Building the libc++ separately, with the host compiler, also decreases the maximum amount of disk space the CI jobs need.

In terms of host compiler, the libc++ being used requires [apple-]clang >= 19 or GCC 15. The other compilers are still built in the pre-existing manner, using the bootstrap build.

For now, GCC 15 doesn't use this yet, because there is some issue linking the library which needs to be investigated.

apple-clang, when built with sanitizers, also has a special requirement which is not being met, so is also not opting into the standalone libc++ build. This is left to improve later.

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[alandefreitas]

I assume there will be no problem

#950 (comment)

these are orthogonal

Only if bootstrap is not a source of truth, which it attempts to be

In fact, one future plan is to replace literally everything in CI with a single call to bootstrap.py

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[alandefreitas]

An automated preview of the documentation is available at https://950.mrdocs.prtest2.cppalliance.org/index.html

@sdarwin, is it possible to tell the bot to only make this comment once per PR?

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[sdarwin]

@alandefreitas reviewing the available settings in Jenkins, there is a possibility: "Ignore Pull Requests marked as Drafts". Coincidentally, a pull request that's in very active development, getting 100 updates, is probably a "draft", right? It's not ready to merge.
Set this PR 950 to be a "draft" and lets see if previews stop.

[mizvekov]

@alandefreitas reviewing the available settings in Jenkins, there is a possibility: "Ignore Pull Requests marked as Drafts". Coincidentally, a pull request that's in very active development, getting 100 updates, is probably a "draft", right? It's not ready to merge. Set this PR 950 to be a "draft" and lets see if previews stop.

It's not possible to turn PRs back into drafts on GitHub.

FWIW this shouldn't have to rely on that, specially in this case where the url doesn't even change.

I have seen similar bot actions, where even when there is an actual update, the bot just updates the existing comment in place, instead of adding a new comment.

[sdarwin]

It's not possible to turn PRs back into drafts

It's now a draft.

[mizvekov]

TIL you can do that, where is the option?

[sdarwin]

where is the option

Not sure what your current collaboration settings are, if "administrator" (as Alan) there's a "convert to draft" setting near "Reviews".

[alandefreitas]

It's now a draft.

Phew