[ArgPromotion] Set debug location at updated callsites

[ArgPromotion] Set debug location at updated callsites

Set the correct debug location on instructions which load arguments in
preparation for a call to an arg-promoted function.

This prevents location cascade from misattributing the line/scope of one
of these loads to the location of the instruction preceding the call.

Differential Revision: https://reviews.llvm.org/D60113

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@357500 91177308-0d34-0410-b5e6-96231b3b80d8