Fix -Woperator-whitespace for consym (part of #19372)

Due to an oversight, the initial specification and implementation of
-Woperator-whitespace focused on varsym exclusively and completely
ignored consym.

This meant that expressions such as "x+ y" would produce a warning,
while "x:+ y" would not.

The specification was corrected in ghc-proposals pull request #404,
and this patch updates the implementation accordingly.

Regression test included.