coverage: Infer `instances_used` from `pgo_func_name_var_map`

In obscure circumstances, we would sometimes emit a covfun record for a
function with no physical coverage counters, causing `llvm-cov` to fail with
the cryptic error message:

    malformed instrumentation profile data: function name is empty

We can eliminate this mismatch by removing `instances_used` entirely, and
instead inferring its contents from the keys of `pgo_func_name_var_map`.

This makes it impossible for a "used" function to lack a PGO name entry.

0 files changed, 0 insertions, 0 deletions