[Coverage] Ignore 'unused' functions with non-zero execution counts

Frontends emit 'unused' coverage mapping records for functions which are
provably unused in a TU. These unused records contain a single counter
with CounterKind::Zero. However, a function may be unused in one TU and
used in another. When this happens, prefer the records with a full set
of counters instead of arbitrarily picking the first loaded record.

There is no impact on the single-TU case. In the multiple-TU case, this
resolves issues causing a function to appear unused when it's not.

Testing: check-{llvm,clang,compiler-rt}

rdar://42981322

llvm-svn: 339194

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