[WinEH] Rename CatchReturnInst::getParentPad, NFC

Summary:
Rename to getCatchSwitchParentPad, to make it more clear which ancestor
the "parent" in question is.  Add a comment pointing out the key feature
that the returned pad indicates which funclet contains the successor
block.

Reviewers: rnk, andrew.w.kaylor, majnemer

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D16222

llvm-svn: 257933