Support SCC pragmas in declaration context

Not having SCCs at the top level is becoming annoying real quick. For
simplest cases, it's possible to do this transformation:

    f x y = ...
    =>
    f = {-# SCC f #-} \x y -> ...

However, it doesn't work when there's a `where` clause:

    f x y = <t is in scope>
      where t = ...
    =>
    f = {-# SCC f #-} \x y -> <t is out of scope>
      where t = ...

Or when we have a "equation style" definition:

    f (C1 ...) = ...
    f (C2 ...) = ...
    f (C3 ...) = ...
    ...

(usual solution is to rename `f` to `f'` and define a new `f` with a
`SCC`)

This patch implements support for SCC annotations in declaration
contexts. This is now a valid program:

    f x y = ...
      where
        g z = ...
        {-# SCC g #-}
    {-# SCC f #-}

Test Plan: This passes slow validate (no new failures added).

Reviewers: goldfire, mpickering, austin, bgamari, simonmar

Reviewed By: bgamari, simonmar

Subscribers: simonmar, thomie, mpickering

Differential Revision: https://phabricator.haskell.org/D2407