[Coverage] Move logic to encode filenames and mappings into llvm (NFC)

Currently, frontends which emit source-based code coverage have to
duplicate logic to encode filenames and raw coverage mappings properly.
This violates an abstraction layer and forces frontends to copy tricky
code.

Introduce llvm::coverage::encodeFilenamesAndRawMappings() to take care
of this.

This will help us experiment with zlib-compressing coverage mapping
data.

llvm-svn: 273055