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

[Mehdi Amini via cfe-commits]

joker-eph wrote:

> So I think keeping the current behavior is fine, though we might want to introduce an extra option for that in the future (as I can imagine that some people want the opposite, and maximize dylib reuse).

Right now this is controlled by `-DLLVM_ENABLE_PROJECTS=` : if you list `clang` it will import the minimal MLIR dependencies in Clang itself (kind of vendoring the subset of MLIR that clang needs). If instead you list `clang;mlir` then we build all of MLIR and clang depends on MLIR.
At least this is the intent, but not all the configuration have been tested yet (as you found with the dylib).

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