When printing a source line as part of a diagnostic, the source line

might be wider than we're supposed to print. In this case, we try to
select the "important" subregion of the source line, which contains
everything that we want to show (e.g., with underlining and the caret
itself) and tries to also contain some of the context. 

From the fantastically long line in the test case, we get an error
message that slices down to this:

message-length.c:18:120: warning: comparison of distinct pointer types
      ('int *' and 'float *')
  a_func_to_call(ip == FloatPointer, ip[ALongIndexName], 
                 ~~ ^  ~~~~~~~~~~~~

There are a bunch of gee-it-sounds-good heuristics in here, which seem
to do well on the various simple tests I've thrown at it. However,
we're going to need to look at a bunch more diagnostics to tweak these
heuristics.

This is the second part of <rdar://problem/6711348>. Almost there! 

llvm-svn: 70597