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