[WinEH] Verify consistent funclet unwind exits

Summary:
A funclet EH pad may be exited by an unwind edge, which may be a
cleanupret exiting its cleanuppad, an invoke exiting a funclet, or an
unwind out of a nested funclet transitively exiting its parent.  Funclet
EH personalities require all such exceptional exits from a given funclet to
have the same unwind destination, and EH preparation / state numbering /
table generation implicitly depends on this.  Formalize it as a rule of
the IR in the LangRef and verifier.


Reviewers: rnk, majnemer, andrew.w.kaylor

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D15962

llvm-svn: 257273