[llvm] Run delta-pass with child process (PR #87647)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> > I think this is a bad direction to go in. If there are crashes in the reductions themselves, we should just fix them. This is just papering over real issues
> 
> but papering over real issues is useful. for example if I'm reducing something that triggers a crash in simplifyCFG, then llvm-reduce is going to tend to crash when it runs simplifyCFG. if we have a super easy workaround for this, we'll get a reduced test case faster, and then we'll get the bug fixed faster.

But how often is this actually happening? Fixing crashes on some unhandled IR is easy, and I can't recall ever encountering a reduction crashing 

> 
> as long as this is turned off by default, we're only papering over problems when we need to do so. I'm in favor of a patch that does this.



https://github.com/llvm/llvm-project/pull/87647