[lld/LTO] Remove unused Path parameter to AddBufferFn

Summary:
With D43396, no clients use the Path parameter anymore.
This is the lld side fix with D43400.

Depends on D43396 and D43400.

Reviewers: pcc

Subscribers: emaste, inglorion, arichardson, llvm-commits

Differential Revision: https://reviews.llvm.org/D43401

llvm-svn: 325620