[LangRef] Add elementtype attribute

This adds an elementtype(<ty>) attribute, which can be used to
attach an element type to a pointer typed argument. It is similar
to byval/byref in purpose, but unlike those does not carry any
specific semantics by itself. However, certain intrinsics may
require it and interpret it in specific ways.

The in-tree use cases for this that I'm currently aware of are:

    call ptr @llvm.preserve.array.access.index.p0.p0(ptr elementtype(%ty) %base, i32 %dim, i32 %index)
    call ptr @llvm.preserve.struct.access.index.p0.p0(ptr elementtype(%ty) %base, i32 %gep_index, i32 %di_index)
    call token @llvm.experimental.gc.statepoint.p0(i64 0, i32 0, ptr elementtype(void ()) @foo, i32 0, i32 0, i32 0, i32 0, ptr addrspace(1) %obj)

Notably, the gc.statepoint case needs a function as element type,
in which case the workaround of adding a separate %ty undef
argument would not work, as arguments cannot be unsized.

Differential Revision: https://reviews.llvm.org/D105407