[X86] Don't give catch objects a displacement of zero

Catch objects with a displacement of zero do not initialize a catch
object.  The displacement is relative to %rsp at the end of the
function's prologue for x86_64 targets.

If we place an object at the top-of-stack, we will end up wit a
displacement of zero resulting in our catch object remaining
uninitialized.

Address this by creating our catch objects as fixed objects.  We will
ensure that the UnwindHelp object is created after the catch objects so
that no catch object will have a displacement of zero.

Differential Revision: http://reviews.llvm.org/D17823

llvm-svn: 262546