[WinEH] Recognize SEH finally block inserted by the frontend

This allows winehprepare to build sensible llvm.eh.actions calls for SEH
finally blocks.  The pattern matching in this change is brittle and
should be replaced with something more robust soon.  In the meantime,
this will let us write the code that produces __C_specific_handler xdata
tables, which we need regardless of how we decide to get finally blocks
through EH preparation.

llvm-svn: 234663