[WinEH] Don't let UnwindHelp alias the return address

[WinEH] Don't let UnwindHelp alias the return address

On top of that, don't bother allocating and initializing UnwindHelp if
we don't have any funclets. Currently we always use RBP as our frame
pointer when funclets are present, so this change makes it impossible to
come here without any fixed stack objects.

Fixes PR25533.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@253245 91177308-0d34-0410-b5e6-96231b3b80d8