[clang] [llvm] [llvm][clang] Allocate a new stack instead of spawning a new thread to get more stack space (PR #133173)

[via llvm-commits]

cor3ntin wrote:

> > If split stacks are negatively impacting profiling, debugging, or other compiler development tasks, I have to ask, have we considered optimizing clang stack usage?
> 
> Strong +1 to this.

I removed a few uses of ParsedAttributes recently, it helped https://github.com/llvm/llvm-project/pull/132021#issue-2931754944 - I'm sure there is a lot more room for improvement

I wish we had some way to profile whether our SmallVectors and other SSO optimizations are reasonably dimensioned.
We also store references to Sema in a ton of places where, realistically, Sema might as well be a singleton. I wonder if we do that enough to have a measurable impact on stack size.


But anyway, we still need a way to grow the stack :)

https://github.com/llvm/llvm-project/pull/133173