Fix desugaring of pattern bindings (again)
This patch fixes Trac #12595. The problem was with a pattern binding like !x = e For a start it's silly to match that pattern and build a unit tuple (the General Case of mkSelectorBinds); but that's what was happening because the bang fell through to the general case. But for a variable pattern building any auxiliary bindings is stupid. So the patch introduces a new case in mkSelectorBinds for variable patterns. Then it turned out that if 'e' was a plain variable, and moreover was imported GlobalId, then matchSinglePat made it a /bound/ variable, which should never happen. That ultimately caused a linker error, but the original bug was much earlier.