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

[Petr Hosek via llvm-commits]

petrhosek wrote:

In LLVM, we have runtimes and bootstrapping builds.

The _runtimes build_ uses https://github.com/llvm/llvm-project/tree/main/runtimes to build runtimes specified by `LLVM_ENABLE_RUNTIMES` using whatever host compiler you specify by `CMAKE_{C,CXX}_COMPILER`.

The _bootstrapping build_ uses https://github.com/llvm/llvm-project/tree/main/llvm/runtimes to first build a toolchain with projects specified by `LLVM_ENABLE_PROJECTS`, and then uses that just built toolchain to build selected runtimes using the _runtimes build_.

This patch only sets `LLVM_RUNTIME_<project>_BUILD` in the _bootstrapping build_ which is not the right place, it should be set in the _runtimes build_ (that is https://github.com/llvm/llvm-project/blob/main/runtimes/CMakeLists.txt) to make sure that the variable is set whether we're using runtimes or bootstrapping build.

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