[DwarfEhPrepare] Assign dummy debug location for inserted _Unwind_Resume calls (PR57469)

DwarfEhPrepare inserts calls to _Unwind_Resume into landing pads.
If _Unwind_Resume happens to be defined in the same module and
debug info is used, then this leads to a verifier error:

  inlinable function call in a function with debug info must
    have a !dbg location
  call void @_Unwind_Resume(ptr %exn.obj) #0

Fix this by assigning a dummy location to the call. (As this
happens in the backend, inlining is not actually relevant here.)

Fixes https://github.com/llvm/llvm-project/issues/57469.

Differential Revision: https://reviews.llvm.org/D133095