[WinEH] Make llvm.eh.actions use frameescape indices for catch params

This makes it possible to use the same representation of llvm.eh.actions
in outlined handlers as we use in the parent function because i32's are
just constants that can be copied freely between functions.

I had to add a sentinel alloca to the list of child allocas so that we
don't try to sink the catch object into the handler. Normally, one would
use nullptr for this kind of thing, but TinyPtrVector doesn't support
null elements. More than that, it's elements have to have a suitable
alignment. Therefore, I settled on this for my sentinel:

  AllocaInst *getCatchObjectSentinel() {
    return static_cast<AllocaInst *>(nullptr) + 1;
  }

llvm-svn: 233947