Hello-

PR preprocessor/49973 addresses the fact that the column number in diagnostics
is not correct if the source lines contain multibyte characters. The attached
patch corrects this. The outlines of it were discussed starting here:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=49973#c13. If it looks OK, could
you please apply it? The patch is a bit large unfortunately, because the column
number was just the tip of the iceberg. There is a lot of machinery in
diagnostic-show-locus.c that also requires multibyte awareness, which I have
done in this patch as well.

A couple notes: 
    - In order to avoid any portability problems with wchar_t, the
equivalent of wcwidth() from libc is implemented in-house.

    - The diagnostic-show-locus stuff needs to know both the bytes column and
the display column. All location infrastructure (e.g. line-map.h) tracks byte
offsets, so I left that all alone. Rather, diagnostic-show-locus.c just
computes display columns on demand when it needs them.

    - In the above-linked PR, Joseph and I also discussed a couple related
fixes that need to be made, namely, when -finput-charset is in use, the
translation needs to be applied when reading source lines for the purpose of
diagnostics printing, which currently it is not, and also, when outputting
diagnostics, the current locale should be inspected to avoid outputting bytes
that the user's terminal cannot display. I have separate patches for these two
as well, but since they are more or less orthogonal to this one, I will submit
them later.

Thank you for your time taking a look at this. It will be nice to clean these
up as part of the support for extended characters in identifiers. I have a
related issue for extended characters also in PR 91843
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91843#c1). I can also submit that
couple-line patch here if it looks OK.

-Lewis