[AMDGPU] Change section name with metadata access

Summary:
The commit rL348922 introduced a means to set Metadata
section kind for a global variable, if its explicit section
name was prefixed with ".AMDGPU.metadata.".

This patch changes that prefix to ".AMDGPU.comment.",
as "metadata" in the section name might lead to
ambiguity with metadata used by AMD PAL runtime.

Change-Id: Idd4748800d6fe801441d91595fc21e5a4171e668

Reviewers: kzhuravl

Reviewed By: kzhuravl

Subscribers: arsenm, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D56197

llvm-svn: 350292