[llvm] [CI] Clean up runtimes builds (PR #131913)

[Michael Kruse via llvm-commits]

Meinersbur wrote:

> > [libc](https://github.com/llvm/llvm-project/blob/main/.github/workflows/libc-fullbuild-tests.yml?rgh-link-date=2025-03-20T16%3A15%3A45Z) and [libcxx](https://github.com/llvm/llvm-project/blob/main/.github/workflows/libcxx-build-and-test.yaml?rgh-link-date=2025-03-20T16%3A15%3A45Z) already have their own pre-merge tests. They use stock compilers (gcc and clang), so the additional value here is only whether Clang is regressing. If worker load is at capacity, I would even argue there is not need to bootstap-build libcxx/libc at all, for the same reason a 2-stage build of Clang is probably not worth it: A littlle more coverage, but whether can build in principle has already been verified.
> 
> I think that "whether Clang is regressing" is a significant value, given how many regressions we're seeing: [llvm/llvm-project/issues (state:open label:clang,clang:frontend,clang:PCH,clang:modules,clang:diagnostics,clang:driver,clang:codegen,clang:headers,clang:analysis label:regression,regression:20,regression:19,regression:18,regression:17,regression:16)](https://github.com/llvm/llvm-project/issues?q=sort%3Aupdated-desc+is%3Aissue+state%3Aopen+label%3Aclang%2Cclang%3Afrontend%2Cclang%3APCH%2Cclang%3Amodules%2Cclang%3Adiagnostics%2Cclang%3Adriver%2Cclang%3Acodegen%2Cclang%3Aheaders%2Cclang%3Aanalysis+label%3Aregression%2Cregression%3A20%2Cregression%3A19%2Cregression%3A18%2Cregression%3A17%2Cregression%3A16).

That's not the point. There is still the buildbot CI infrastructure that would hopefully catch these regressions. If we have infinite resources, we should pre-merge check everything. But since the point of this PR is that there is not, a tradeoff must be made. I am arguing that having each source file compiled in at least one configuration would be reasonable, every additional configuration has diminishing returns.

If I could wish for another configuration that I would like to test, it would be `BUILD_SHARED_LIBS=ON`. Builds fail regularly because a dependency has been [forgotten](https://github.com/llvm/llvm-project/commit/1ca93b15482d3bfa1560b35960ab46fea65b3074), causing a subsequent [fix](https://github.com/llvm/llvm-project/commit/a685045b9fb4218a4a57f765b0642c3074eebdd8)/[revert](https://github.com/llvm/llvm-project/commit/2ec6174bef4bc9ef3d5cedbffd7169017c9669c3). The point of the Pre-Merge over Post-Commit CI seems to avoid exactly this churn, not that otherwise these would not be caught. There are literally [premerge-monolithic-linux](https://lab.llvm.org/buildbot/#/builders/153)/[premerge-monolithic-windows](https://lab.llvm.org/buildbot/#/builders/35) post-build configurations that should catch these.

https://github.com/llvm/llvm-project/pull/131913