introduce a new CharSourceRange class, and enhance the diagnostics routines

introduce a new CharSourceRange class, and enhance the diagnostics routines
to use them instead of SourceRange.  CharSourceRange is just a SourceRange
plus a bool that indicates whether the range has the end character resolved
or whether the end location is the start of the end token.  While most of
the compiler wants to think of ranges that have ends that are the start of
the end token, the printf diagnostic stuff wants to highlight ranges within
tokens.

This is transparent to the diagnostic stuff.  To start taking advantage of
the new capabilities, you can do something like this:
  Diag(..) << CharSourceRange::getCharRange(Begin,End)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@106338 91177308-0d34-0410-b5e6-96231b3b80d8