This commit ensures that we propagate the enclosing long distance
information to let bindings inside guards, in order to get accurate
pattern-match checking warnings, in particular incomplete record
selector warnings.

Example:

  data D = K0 | K1 { fld :: Int }
  f :: D -> Int
  f d@(K1 {})
    | let i = fld d
    = i
  f _ = 3

We now correctly recognise that the field selector 'fld' cannot fail,
due to the outer pattern match which guarantees that the value 'd' has
the field 'fld'.

Fixes #25749