Previously modules with hscTarget == HscNothing were not desugared.
This patch changes behavior so that all modules HsSrcFile Modules except GHC.Prim
are desugared. Modules with hscTarget == HscNothing are not simplified.

Warnings and errors produced by the desugarer will now be produced when
compiling with -fno-code.

HscMain.finishTypecheckingOnly is removed, HscMain.hscIncrementalCompile is
simplified a bit, and HscMain.finish takes in the removed logic. I think this
is easier to follow.

Updates haddock submodule.

Tests T8101, T8101b, T10600 are no longer expect_broken.

Reviewers: ezyang, austin, bgamari

Subscribers: rwbarton, thomie

GHC Trac Issues: #10600

Differential Revision: https://phabricator.haskell.org/D3542