[llvm-cov] Fix illegal cast from uint64_t to int64_t

Summary:
Counters are stored as uint64_t in the coverage mapping, but
exporting in JSON requires signed integers. Clamp the values to the
smaller range to make the conversion safe.

Reviewers: Dor1s, vsk

Reviewed By: Dor1s

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70200