Fix infix record field fixity (#11167 and #11173).

This extends D1585 with proper support for infix duplicate record
fields.  In particular, it is now possible to declare record fields as
infix in a module for which `DuplicateRecordFields` is enabled, fixity
is looked up correctly and a readable (although unpleasant) error
message is generated if multiple fields with different fixities are in
scope.

As a bonus, `DEPRECATED` and `WARNING` pragmas now work for
duplicate record fields. The pragma applies to all fields with the
given label.

In addition, a couple of minor `DuplicateRecordFields` bugs, which were
pinpointed by the `T11167_ambig` test case, are fixed by this patch:

  - Ambiguous infix fields can now be disambiguated by putting a type
    signature on the first argument

  - Polymorphic type constructor signatures (such as `ContT () IO a` in
    `T11167_ambig`) now work for disambiguation

Parts of this patch are from D1585 authored by @KaneTW.

Test Plan: New tests added.

Reviewers: KaneTW, bgamari, austin

Reviewed By: bgamari

Subscribers: thomie, hvr

Differential Revision: https://phabricator.haskell.org/D1600

GHC Trac Issues: #11167, #11173

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