This MR is an implementation of the proposal #516.

It adds a warning -Wincomplete-record-selectors for usages of a record
field access function (either a record selector or getField @"rec"),
while trying to silence the warning whenever it can be sure that a constructor
without the record field would not be invoked (which would otherwise cause
the program to fail). For example:

    data T = T1 | T2 {x :: Bool}

    f a = x a -- this would throw an error

    g T1 = True
    g a = x a -- this would not throw an error

    h :: HasField "x" r Bool => r -> Bool
    h = getField @"x"

    j :: T -> Bool
    j = h -- this would throw an error because of the `HasField`
          -- constraint being solved

See the tests DsIncompleteRecSel* and TcIncompleteRecSel for more examples of the warning.
See Note [Detecting incomplete record selectors] in GHC.HsToCore.Expr for implementation details