...in preparation for non-source locations, i.e. locations that don't come
frome source buffers.
No functionality change, but a fair bit of SourceManager API and idioms have
changed.
Swift SVN r18942
integration
Motivation: libIDE clients should be simple, and they should not have to
translate token-based SourceRanges to character locations.
This also allows us to remove the dependency of DiagnosticConsumer on the
Lexer. Now the DiagnosticEngine translates the diagnostics to CharSourceRanges
and passes character-based ranges to the DiagnosticConsumer.
Swift SVN r7173