bug#49624: compilation message end-column function off-by-one bug

[Mattias Engdegård]

Compilation message patterns (in compilation-regexp-alist and -alist) can 
indicate starting and ending line and column numbers, either by supplying 
regexp match numbers or functions that return the respective line/column 
numbers when called. In other words, the integer N can be understood as a 
shorthand for the function

(lambda () (and (match-beginning N) (string-to-number (match-string N))))

except that isn't true for the ending column where there is a difference of 1; 
an END-COL function returning 13 means that the error's last column is 12.

There does not seem to be a good justification for this nor is it documented so 
it's probably just a bug; proposed patch attached below. Does it warrant a 
mention in etc/NEWS?

0001-Fix-off-by-one-error-in-compilation-rule-end-column-.patch
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