This commit implements the ability to deprecate certain instances,
which causes the compiler to emit the desired deprecation message
whenever they are instantiated. For example:

  module A where
  class C t where
  instance {-# DEPRECATED "dont use" #-} C Int where

  module B where
  import A
  f :: C t => t
  f = undefined
  g :: Int
  g = f -- "dont use" emitted here

The implementation is as follows:
  - In the parser, we parse deprecations/warnings attached to instances:

      instance {-# DEPRECATED "msg" #-} Show X
      deriving instance {-# WARNING "msg2" #-} Eq Y

    (Note that non-standalone deriving instance declarations do not support
    this mechanism.)

  - We store the resulting warning message in `ClsInstDecl` (respectively, `DerivDecl`).
    In `GHC.Tc.TyCl.Instance.tcClsInstDecl` (respectively, `GHC.Tc.Deriv.Utils.newDerivClsInst`),
    we pass on that information to `ClsInst` (and eventually store it in `IfaceClsInst` too).

  - Finally, when we solve a constraint using such an instance, in
    `GHC.Tc.Instance.Class.matchInstEnv`, we emit the appropriate warning
    that was stored in `ClsInst`.
    Note that we only emit a warning when the instance is used in a different module
    than it is defined, which keeps the behaviour in line with the deprecation of
    top-level identifiers.

Signed-off-by: Bartłomiej Cieślar <bcieslar2001@gmail.com>