Note [TcLevel of ExpType]

Consider

  data G a where
    MkG :: G Bool

  foo MkG = True

This is a classic untouchable-variable / ambiguous GADT return type
scenario. But, with ExpTypes, we'll be inferring the type of the RHS.
We thus must track a TcLevel in an Inferring ExpType. If we try to
fill the ExpType and find that the TcLevels don't work out, we fill
the ExpType with a tau-tv at the low TcLevel, hopefully to be worked
out later by some means -- see fillInferResult, and Note [fillInferResult]

This behaviour triggered in test gadt/gadt-escape1.