[WinEH] Verify unwind edges against EH pad tree

Summary:
Funclet EH personalities require a tree-like nesting among funclets
(enforced by the ParentPad linkage in the IR), and also require that
unwind edges conform to certain rules with respect to the tree:
 - An unwind edge may exit 0 or more ancestor pads
 - An unwind edge must enter exactly one EH pad, which must be distinct
   from any exited pads
 - A cleanupret's edge must exit its cleanuppad

Describe these rules in the LangRef, and enforce them in the verifier.


Reviewers: rnk, majnemer, andrew.w.kaylor

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D15961

llvm-svn: 257272