[flang] Use of precompiled headers leads to undetected missing includes

[mgorny]

I do realize that @mrkajetanp did introduce precompiled headers to reduce memory usage on my request in #131137, but looks like we missed a major problem with them: they hide missing includes, so now buildbots remain green when a code with missing includes is committed, but the end user builds without PCH fail.

Perhaps we should disable PCH on (some of the) buildbots via -DCMAKE_DISABLE_PRECOMPILE_HEADERS=ON to catch issues.

See #143196

[mrkajetanp]

I think you're right about what the solution should be. What if we just disable PCH on the GitHub pre-merge CI? It'd be a lot better to catch these things out early rather than have to fix them after the fact.

[DavidSpickett]

If the problem is the local build working but the bot not, it would be best to report this in pre-commit. It's fine for me to get a post-commit notification because I'm contributing all the time, but another cycle is a bigger burden for occasional contributors.

Post-commit is always nice to have, because maybe we miss a CI dependency or push something directly.

We (Linaro) had reports of compilation errors on Windows and are going to fix our buildbot configurations to disable PCH for that reason. This is hopefully not permanent though.

For Linux, we have the budget to build without PCH.

How wide does the coverage need to be? Includes mostly vary by platform I'd guess. So is one "standard" Linux and Windows configuration enough?

Aside from Linaro's bots I see various Linux variants, Red Hat, SLES, then there is AIX as well.