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

[Erich Keane via cfe-commits]

erichkeane wrote:

> > > At the moment it looks between 1-2%, would there be an acceptable increase, or does the artifact size make it just completely untenable?
> > 
> > 
> > A 1-2% increase in startup time would be unproblematic -- what I'm actually seeing in local tests is a ~20% increase in wall-time when running clang on `int main() { return 0; }` (which should be about representative of `./configure` use cases...) with a dylib release build.
> 
> Interesting, I am not seeing the 20% increase, but I'm deep into trying to mess with CMake to minimize the startup time. I'd be interested in seeing if the patch I'm working on makes a difference if you'd be able/willing to test it.

I did this one: https://github.com/llvm/llvm-project/pull/232409 

It reduces the overhead at load time to a reasonable amount as far as I can tell, but I'd love to see if it satisifies your concerns (regarding run-time).

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