[IR] Partially remove pointer element types from intrinsic signatures (NFC)

[IR] Partially remove pointer element types from intrinsic signatures (NFC)

As typed pointers are no longer supported, we should no longer
specify element types in intrinsic signatures.

The only meaningful pointer types are now:

    llvm_ptr_ty -> ptr
    llvm_anyptr_ty -> ptr addrspace(any)
    LLVMQualPointerType<N> -> ptr addrspace(N)

This is only "partially" because we also have a bunch of special
IIT descriptors like LLVMPointerTo, LLVMPointerToElt and
LLVMAnyPointerToElt, which I'll leave for a later revision.

Differential Revision: https://reviews.llvm.org/D155086