[clang] [llvm] Enable ClangIR Build By Default (PR #221291)

[Erich Keane via cfe-commits]

erichkeane wrote:

> Can you please make sure this works with the dylib build?
> 
> Currently this fails to link for the full build, and if you build just clang the binary is completely broken.

Would just adding `-DLLVM_BUILD_LLVM_DYLIB=ON` to cmake be sufficient to test this here (plus testing clang?)?  Or is there more to dynlib than that? 

https://github.com/llvm/llvm-project/pull/221291