llvm-cov: Move some reader debug output out of the tool.

This debug output is really for testing CoverageMappingReader, not the
llvm-cov tool. Move it to where it can be more useful.
llvm-cov: Disentangle the coverage data logic from the display (NFC)

This splits the logic for actually looking up coverage information
from the logic that displays it. These were tangled rather thoroughly
so this change is a bit large, but it mostly consists of moving things
around. The coverage lookup logic itself now lives in the library,
rather than being spread between the library and the tool.