Allow fix_coverage.py to work on gcov output too

The format for gcov output is described in the [llvm-cov documentation](https://llvm.org/docs/CommandGuide/llvm-cov.html#id2) (last line of the description for `llvm-cov gcov`). It shouldn't be too challenging to adjust fix_coverage.py to detect this format and replace `-`s with `####`s on templated lines.

Read fix_coverage.py and the linked llvm-cov gcov format description first. Identify how templated lines are currently recognized, then support gcov output by detecting its format and replacing '-' with '####' on templated lines. Done means gcov reports are handled without regressing existing behavior.

Written by the indexing model from the issue text.