Fix unused-import warnings

This patch fixes a fairly long-standing bug (dating back to 2015) in
RdrName.bestImport, namely

   commit 9376249b6b78610db055a10d05f6592d6bbbea2f
   Author: Simon Peyton Jones <simonpj@microsoft.com>
   Date:   Wed Oct 28 17:16:55 2015 +0000

   Fix unused-import stuff in a better way

In that patch got the sense of the comparison back to front, and
thereby failed to implement the unused-import rules described in
  Note [Choosing the best import declaration] in RdrName

This led to Trac #13064 and #15393

Fixing this bug revealed a bunch of unused imports in libraries;
the ones in the GHC repo are part of this commit.

The two important changes are

* Fix the bug in bestImport

* Modified the rules by adding (a) in
     Note [Choosing the best import declaration] in RdrName
  Reason: the previosu rules made Trac #5211 go bad again.  And
  the new rule (a) makes sense to me.

In unravalling this I also ended up doing a few other things

* Refactor RnNames.ImportDeclUsage to use a [GlobalRdrElt] for the
  things that are used, rather than [AvailInfo]. This is simpler
  and more direct.

* Rename greParentName to greParent_maybe, to follow GHC
  naming conventions

* Delete dead code RdrName.greUsedRdrName

Bumps a few submodules.

Reviewers: hvr, goldfire, bgamari, simonmar, jrtc27

Subscribers: rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5312

5 files changed, 0 insertions, 6 deletions

diff --git a/compiler/nativeGen/RegAlloc/Linear/JoinToTargets.hs b/compiler/nativeGen/RegAlloc/Linear/JoinToTargets.hs
index fb002e2fe0..546d48af21 100644
--- a/compiler/nativeGen/RegAlloc/Linear/JoinToTargets.hs
+++ b/compiler/nativeGen/RegAlloc/Linear/JoinToTargets.hs
@@ -27,8 +27,6 @@ import Unique
 import UniqFM
 import UniqSet
 
-import Data.Foldable (foldl')
-
 -- | For a jump instruction at the end of a block, generate fixup code so its
 --      vregs are in the correct regs for its destination.
 --
diff --git a/compiler/nativeGen/RegAlloc/Linear/PPC/FreeRegs.hs b/compiler/nativeGen/RegAlloc/Linear/PPC/FreeRegs.hs
index 581548212a..24577c446c 100644
--- a/compiler/nativeGen/RegAlloc/Linear/PPC/FreeRegs.hs
+++ b/compiler/nativeGen/RegAlloc/Linear/PPC/FreeRegs.hs
@@ -13,7 +13,6 @@ import Platform
 
 import Data.Word
 import Data.Bits
-import Data.Foldable (foldl')
 
 -- The PowerPC has 32 integer and 32 floating point registers.
 -- This is 32bit PowerPC, so Word64 is inefficient - two Word32s are much
diff --git a/compiler/nativeGen/RegAlloc/Linear/SPARC/FreeRegs.hs b/compiler/nativeGen/RegAlloc/Linear/SPARC/FreeRegs.hs
index 653b2707c9..09003cf0a3 100644
--- a/compiler/nativeGen/RegAlloc/Linear/SPARC/FreeRegs.hs
+++ b/compiler/nativeGen/RegAlloc/Linear/SPARC/FreeRegs.hs
@@ -15,7 +15,6 @@ import Platform
 
 import Data.Word
 import Data.Bits
-import Data.Foldable (foldl')
 
 
 --------------------------------------------------------------------------------
diff --git a/compiler/nativeGen/RegAlloc/Linear/X86/FreeRegs.hs b/compiler/nativeGen/RegAlloc/Linear/X86/FreeRegs.hs
index 65a566d1c3..b8af046d82 100644
--- a/compiler/nativeGen/RegAlloc/Linear/X86/FreeRegs.hs
+++ b/compiler/nativeGen/RegAlloc/Linear/X86/FreeRegs.hs
@@ -13,7 +13,6 @@ import Platform
 
 import Data.Word
 import Data.Bits
-import Data.Foldable (foldl')
 
 newtype FreeRegs = FreeRegs Word32
     deriving Show
diff --git a/compiler/nativeGen/RegAlloc/Linear/X86_64/FreeRegs.hs b/compiler/nativeGen/RegAlloc/Linear/X86_64/FreeRegs.hs
index 713b053356..3a6e3407a7 100644
--- a/compiler/nativeGen/RegAlloc/Linear/X86_64/FreeRegs.hs
+++ b/compiler/nativeGen/RegAlloc/Linear/X86_64/FreeRegs.hs
@@ -11,7 +11,6 @@ import Reg
 import Panic
 import Platform
 
-import Data.Foldable (foldl')
 import Data.Word
 import Data.Bits