PmCheck: Disattach COMPLETE pragma lookup from TyCons

By not attaching COMPLETE pragmas with a particular TyCon and instead assume that every COMPLETE pragma is applicable everywhere, we can drastically simplify the logic that tries to initialise available COMPLETE sets of a variable during the pattern-match checking process, as well as fixing a few bugs.

Of course, we have to make sure not to report any of the ill-typed/unrelated COMPLETE sets, which came up in a few regression tests.

In doing so, we fix #17207 (closed), #18277 (closed) and #14422.