The semantics of invoke require that we always jump to the unwind block

The semantics of invoke require that we always jump to the unwind block
(landing pad) when an exception unwinds through the call.  This doesn't
quite match the way the dwarf unwinder works: by default it only jumps to
the landing pad if the catch or filter specification matches, and otherwise
it keeps on unwinding.  There are two ways of specifying to the unwinder
that it should "always" (more on why there are quotes here later) jump to
the landing pad: follow the specification by a 0 typeid, or follow it by
the typeid for the NULL typeinfo.  GCC does the first, and this patch makes
LLVM do the same as gcc.  However there is a problem: the unwinder performs
optimizations based on C++ semantics (it only expects destructors to be
run if the 0 typeid fires - known as "cleanups"), meaning it assumes that no
exceptions will be raised and that the raised exception will be reraised
at the end of the cleanup code.  So if someone writes their own LLVM code
using the exception intrinsics they will get a nasty surprise if they don't
follow these rules.  The other possibility of using the typeid corresponding
to NULL (catch-all) causes the unwinder to make no assumptions, so this is
probably what we should use in the long-run.  However since we are still
having trouble getting exception handling working properly, for the moment
it seems best to closely imitate GCC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@37399 91177308-0d34-0410-b5e6-96231b3b80d8