Initial check in that will auto-upgrade the old EH scheme to the new EH scheme.

This upgrade suffers from the problems of the old EH scheme - i.e., that the
calls to llvm.eh.exception() and llvm.eh.selector() can wander off and get
lost. It makes a valiant effort to reclaim these little lost lambs.

This is a first draft, so it hasn't yet been hooked up to the parser.

llvm-svn: 138602