[clang] Add Bytes/Columns types to TextDiagnostic (llvm#165541)

In `TextDiagnostic.cpp`, we're using column- and byte indices
everywhere, but we were using integers for them which made it hard to
know what to pass where, and what was produced. To make matters worse,
that `SourceManager` considers a "column" is a byte in `TextDiagnostic`.

Add `Bytes` and `Columns` structs, which are not related so API using
them can differentiate between values interpreted columns or bytes.

Author: tbaederr