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

[Reid Kleckner via llvm-commits]

https://github.com/rnk commented:

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

There are multiple issues:
* Clang is a recursive descent parser. This isn't going to change, it just means we can't ignore stack usage.
* We use lots of needlessly nested "small" data structures on the stack, which means are stack frames are large
* Anecdotally I am told that LLVM is not great at stack coloring

This is not a blocking concern, but we should seriously consider doing some builds with -Wframe-larger-than or -Wstack-usage in Sema and Parser. We'd all be happier for it.

I had some comments, but overall this seems reasonable.

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