Fix the roundtrip test under llvm-profdata

Summary:
According to the comments, we want to convert the profile into two binary formats, and then into the md5text format.
We seems to have ignored the intermediate files.
This patch uses them to complete the full roundtrips.

Reviewers: wmi, wenlei

Reviewed By: wmi

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81202