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

[Corentin Jabot via cfe-commits]

https://github.com/cor3ntin commented:

I wonder if it would be useful to add documentation to
https://llvm.org/docs/CMake.html#frequently-used-cmake-variables

When you merge, can you let people know on the rfc and possibly the infrastructure channel?

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