[NFC] SCEVExpander: add SetCurrentDebugLocation() / getCurrentDebugLocation() wrappers

Summary:
The internal `Builder` is private, which means there is
currently no way to set the debuginfo locations for `SCEVExpander`.
This only adds the wrappers, but does not use them anywhere.

Reviewers: mkazantsev, sanjoy, gberry, jyknight, dneilson

Reviewed By: sanjoy

Subscribers: javed.absar, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61007

llvm-svn: 370453