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

[John Regehr via llvm-commits]

regehr 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. 

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