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Previously, GHC would not warn whenever there was a class instance that
didn't implement a class method whose name begins with an underscore.

Fixes #12959.

Test Plan: make test TEST=WarnMinimal

Reviewers: austin, bgamari, simonpj

Reviewed By: bgamari, simonpj

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2849

GHC Trac Issues: #12959

Both gcc and clang tell which warning flag a reported warning can be
controlled with, this patch makes ghc do the same. More generally, this
allows for annotated compiler output, where an optional annotation is
displayed in brackets after the severity.

This also adds a new flag `-f(no-)show-warning-groups` to control
whether to show which warning-group (such as `-Wall` or `-Wcompat`)
a warning belongs to. This flag is on by default.

This implements #10752

Reviewed By: quchen, bgamari, hvr

Differential Revision: https://phabricator.haskell.org/D1943

This matches GCC's choice of Unicode quotation marks (i.e. U+2018 and U+2019)
and therefore looks more familiar on the console. This addresses #2507.
Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>