[llvm-dwarfdump][Statistics] Change the coverage buckets representation. NFC

Summary:
This changes the representation of 'coverage buckets' in llvm-dwarfdump and
llvm-locstats to one that makes more clear what the buckets contain.

See some related details in D71070.

Reviewers: djtodoro, aprantl, cmtice, jhenderson

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71366