Dont emit Mapping symbols for sections that contain only data.

Summary:
Dont emit mapping symbols for sections that contain only data.

Patched by Shankar Easwaran <shankare@codeaurora.org>

Reviewers: rengolin, peter.smith, weimingz, kparzysz, t.p.northover

Reviewed By: t.p.northover

Subscribers: t.p.northover, llvm-commits

Differential Revision: https://reviews.llvm.org/D30724

llvm-svn: 298901