Fix -Woperator-whitespace for consym (part of #19372), 2nd attempt

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.

The first attempt was at !5959 (closed). This time the patch is less invasive.