This patch adds a new pragma so that users can specify `COMPLETE` sets of
`ConLike`s in order to sate the pattern match checker.

A function which matches on all the patterns in a complete grouping
will not cause the exhaustiveness checker to emit warnings.

```
pattern P :: ()
pattern P = ()

{-# COMPLETE P #-}

foo P = ()
```

This example would previously have caused the checker to warn that
all cases were not matched even though matching on `P` is sufficient to
make `foo` covering. With the addition of the pragma, the compiler
will recognise that matching on `P` alone is enough and not emit
any warnings.

Reviewers: goldfire, gkaracha, alanz, austin, bgamari

Reviewed By: alanz

Subscribers: lelf, nomeata, gkaracha, thomie

Differential Revision: https://phabricator.haskell.org/D2669

GHC Trac Issues: #8779