[WinEH] Update coloring to handle nested cases cleanly

Summary:
Change the coloring algorithm in WinEHPrepare to visit a funclet's exits
in its parents' contexts and so properly classify the continuations of
nested funclets.

Also change the placement of cloned blocks to be deterministic and to
maintain the relative order of each funclet's blocks.

Add a lit test showing various patterns that require cloning, the last
several of which don't have CHECKs yet because they require cloning
entire funclets which is NYI.


Reviewers: rnk, andrew.w.kaylor, majnemer

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D12353

llvm-svn: 246245