[llvm] [LLVM] Add `LLVM_<proj>_RUNTIME_TARGETS` to set targets per-project (PR #81557)

[Petr Hosek via llvm-commits]

petrhosek wrote:

I looked at all `"libc" IN_LISTS LLVM_ENABLE_RUNTIMES` cases but neither of those seemed like blockers and could be worked around by setting `LIBC_HDRGEN_ONLY=ON` and `LLVM_ENABLE_PROJECTS=libc` which IMHO is the correct solution rather than complicating the existing logic by iterating over all variables to check for `"libc" IN_LISTS RUNTIMES_<target>_LLVM_ENABLE_RUNTIMES` for every `<target>` in `LLVM_RUNTIME_TARGETS`.

I'm still not especially enthused about this feature, it adds additional complexity to the runtimes build and from what I can tell so far, you can achieve the same result with the existing implementation, and although that might require setting more options, I'm not sure it warrants the additional complexity. If there some specific things you believe aren't possible, it'd be great to provide an example so I can see if we could support those with the existing features.

If we decide to go forward with change regardless, I'd like you to setup a builder that exercises it, ideally both presubmit and postsubmit. The problem we already have is that runtimes build has too many ways to spell the same thing which is making it frustratingly difficult to make changes. For example, in case of libc I have to verify that `LLVM_ENABLE_PROJECTS=libc`, `LLVM_ENABLE_RUNTIMES=libc` and `RUNTIMES_<target>_LLVM_ENABLE_RUNTIMES=libc` still works; with this change there's yet another variation and I don't have resources to try every one of them every time I make some change to the build and I'd like to avoid a situation where I make a change and then hear back some time later that I broke a downstream user.

I'd also be curious to hear from @smeenai and @llvm-beanz as other reviewers of the runtimes build changes if they have other thoughts.

https://github.com/llvm/llvm-project/pull/81557