[RFC] Cleaning and extening the diagnostics machinery

[Benjamin Kosnik]

FWIW, edg does something like pretty-printing for C++ error
messages. It's really, really, really nice. (As an aside, Mark has
indicated in the past that this functionality took a bit of work to
accomplish.)

> The show-location=once setting was a big improvement, as it toned down
> the number of repetitive printings of the filename.

Second. It's been a big help, that and the template parameter compression.

> For highlighting support, personally I'm thinking of GNU ls, where
> ANSI escape sequences can be listed in an LS_COLORS environment
> variable.

Right. The idea is that one could have "Error" or "Warning" in say
red, or italic, or bold, or combinations of these attributes. That
would be a good place to start, but it could be taken to the logical
extreme of highlighting language keywords, line numbers, source files,
etc. (Thus one could configure the messages in such a way that the
language constructs were less noticeable, and the actual warning or
error message was most visible, etc.)

It's a nice thought, isn't it? Perhaps a bit excessive for command
line purists, but for others it could be really handy. Besides,
default behavior doesn't have to change, even with this added
functionality.

-benjamin