[LangRef] Require elementtype attribute for gc.statepoint intrinsic

The gc.statepoint intrinsic currently determines the target function
type based on the pointer element type of the argument. In order to
support opaque pointers, require that the argument is annotated with
an elementtype attribute.

Here's an example of the change:

    ; Before:
      %safepoint_token = tail call token (i64, i32, i1 ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_i1f(i64 0, i32 0, i1 ()* @return_i1, i32 0, i32 0, i32 0, i32 0)

    ; After:
      %safepoint_token = tail call token (i64, i32, i1 ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_i1f(i64 0, i32 0, i1 ()* elementtype(i1 ()) @return_i1, i32 0, i32 0, i32 0, i32 0)

    ; After with opaque pointers:
      %safepoint_token = tail call token (i64, i32, i1 ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0(i64 0, i32 0, ptr elementtype(i1 ()) @return_i1, i32 0, i32 0, i32 0, i32 0)

Differential Revision: https://reviews.llvm.org/D117890