diagnostics: prefer "…" to "..." if the locale supports it
Is this really a win? I normally used fixed-width fonts, and on my
platform a string like "ABC…DEF" is all-too-easily confused with
"ABC_DEF" if there are no underscores in the neighborhood. (I am using
Fedora 30 with default fonts.)
For what it's worth, the Wikipedia manual of style
<https://en.wikipedia.org/wiki/Wikipedia:Manual_of_Style#Ellipses> says,
"Wikipedia's style for an ellipsis is three unspaced dots (...); do not
use the precomposed ellipsis character (…) or three dots separated by
spaces (. . .)." Although Bison is not bound by any such style guide,
I've found the Wikipedia style manual to be a good source for advice
when trying to write clear documentation that is readable on the web.
