[opaque pointer types] Add element type argument to IRBuilder CreatePreserveStructAcc...

[opaque pointer types] Add element type argument to IRBuilder CreatePreserveStructAccessIndex and CreatePreserveArrayAccessIndex

Summary:
These were the only remaining users of the GetElementPtrInst::getGEPReturnType
method that gets the element type from the pointer type.

Remove that method since its now dead.

Reviewers: jyknight, t.p.northover, arsenm

Reviewed By: arsenm

Subscribers: wdng, arsenm, arphaman, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D69756