Don't crash when emitting fixits following Unicode characters.

This code is very sensitive to the difference between "columns" as printed
and "bytes" (SourceManager columns). All variables are now named explicitly
and our assumptions are (hopefully) documented as both comment and assertion.

Whether parseable fixits should use byte offsets or Unicode character counts
is pending discussion on the mailing list; currently the implementation uses
bytes (and has no problems on lines containing multibyte characters).
This has been added to the user manual.

<rdar://problem/11877454>

llvm-svn: 160319