Add some more Win64 EH directives:

- StartChained and EndChained delimit a chained unwind area, which can contain
  additional operations to be undone if an exception occurs inside of it.
- UnwindOnly declares that this function doesn't handle any exceptions. If it
  has a handler, it's an unwind handler instead of an exception handler.
- Lsda declares the location and size of the LSDA, which in the Win64 EH
  scheme is kept inside the UNWIND_INFO struct. Windows itself ignores the
  LSDA; it's used by the Language-Specific Handler (the "Personality Function"
  from DWARF).

llvm-svn: 131572