[WinEH] Don't emit an exceptional cleanup for llvm.eh.endcatch

These extra endcatch markers aren't helping identify regions to outline,
so let's get rid of them.  LLVM outlines (more or less) from begincatch
to endcatch.  Any unwind edge from an enclosed invoke is a transition to
a new exception handler, which has it's own outlining markers.

llvm-svn: 235562