DwarfEHPrepare: insert extra unwind paths for stack protector to instrument

This is a mitigation patch for
https://bugs.chromium.org/p/llvm/issues/detail?id=30, where existing stack
protection is skipped if a function is returned through by an unwinder rather
than the normal call/return path. The recent patch D139254 added the ability to
instrument a visible unwind path, at least in the IR case (I'm working on the
SelectionDAG instrumentation too) but there are still invisible unwinds it
can't reach.

So this patch adds logic to DwarfEHPrepare that goes through a function,
converting any call that might throw into an invoke to a simple resume cleanup,
and adding cleanup clauses to existing landingpads that lack them. Obviously we
don't really want to do this if it's wasted effort, so I also exposed
requiresStackProtector from the actual StackProtector code to skip the extra
paths if they won't be used.

https://reviews.llvm.org/D143637