Refine ArgPromotion metadata handling

Summary:
In r353537 we now copy all metadata to the new function, with the old
being removed when the old function is eliminated. In some cases the old
function is dropped to a declaration (seems to only occur with the old
PM). Go ahead and clear all metadata from the old function to handle that
case, since verification will complain otherwise. This is consistent
with what was being done for debug metadata before r353537.

Reviewers: davidxl, uabelho

Subscribers: jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58215

llvm-svn: 354032