[llvm] [llvm][CMake] Introduce LLVM_RUNTIME_<project>_BUILD in CMake (PR #88934)

[Michael Klemm via llvm-commits]

mjklemm wrote:

> So, is this adding a variable that's used to detect if a project has "any" runtime enabled for it? I know for detecting `libc` stuff it's kind of a mess because we need to check `LLVM_ENABLE_PROJECTS` and `RUNTIMES_<triple>_LLVM_ENABLE_PROJECTS`.

I do hope that this will simply this.  For the OpenMP tests, one would also have to explicitly check the presence of "openmp" in LLVM_ENABLE_PROJECTS.  After this, this will hopefully be not needed anymore.  If you could have a look and see if this PR helps there, I'd appreciate that.

https://github.com/llvm/llvm-project/pull/88934