[llvm] [CMake] Simplify building of bolt-optimized clang for distribution builds (PR #117772)

[Amir Ayupov via llvm-commits]

aaupov wrote:

> As I look into this more, it's not strictly a problem with distribution builds. Just enabling bolt and running `ninja install-clang` does not give you a bolt optimized clang. 

Right, for that matter, it's not triggered by `ninja clang` as well. There's a problem that clang-bolt depends on clang target, so clang-bolt can't be a provider for it. In theory we could decouple targets: if CLANG_BOLT is provided, pre-BOLT clang target could be called differently, and clang-bolt target could be the provider for clang. But I thought it would be too ambitious for me to change how clang is built, which is why clang-bolt is "on the side".

https://github.com/llvm/llvm-project/pull/117772