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

[Andy Kaylor via cfe-commits]

andykaylor wrote:

> We're seeing errors failing to find include files: `fatal error: 'mlir/IR/BuiltinLocationAttributes.h.inc' file not found`. Is anyone else seeing the same thing? Any idea what could be causing it? https://github.com/qualcomm/cpullvm-toolchain/actions/runs/39650133307/job/119395984370

It looks like this was a different issue than the one Erich mentioned. It should be fixed by https://github.com/llvm/llvm-project/pull/234419

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