[llvm-locstats] Fix labels on x-axis of comparison chart

Summary:
This change makes the labels on the x-axis of a comparison chart look
like: "0%", "(0%, 10%)", "[10%, 20%)", and so on.

Previously, each label was the same (a concatenation of all the possible
coverage buckets).

Reviewers: djtodoro

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77155