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

[Joseph Huber via llvm-commits]

jhuber6 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`.

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