This easy-to-make mistake meant that pattern-synonym matcher and
builder Ids weren't being treated as locally defined by the simpplier.
That meant that we never looked up them up in the environment, got an
out-of-date unfolding, which made the Simplifier fall into an infinite
loop.  This was the cause of Trac #98587, but it was quite tricky to
find!

In a separate patch I'll make Lint check for locally-bound GlobalIds,
since they are always an error.