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

[Michael Klemm via llvm-commits]

mjklemm wrote:

> Or, alternatively, even have `LLVM_RUNTIME_<project>_BUILD` and `LLVM_BOOTSTRAP_<project>_BUILD` to distinguish the two variants of these builds to enable other parts of the CMake files to detect either one.

I have implemented thing that way now.  @petrhosek Please have a look and let me know that's an acceptable compromise to be as flexible as possible in detecting how builds are being configured.

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