Enabled warn on tabs by default (fixes #9230)

Summary:
This revision enables -fwarn-tabs by default and add a suppression
flag, so that GHC compilation won't fail when some files contain tab
characters.

Test Plan: Additional test case, T9230, was added to cover that change.

Reviewers: austin

Subscribers: simonmar, ezyang, carter, thomie, mlen

Differential Revision: https://phabricator.haskell.org/D255

GHC Trac Issues: #9230
Fixed remaining tests for #9230

Tests that specify -Wall or -W flags turn -fwarn-tabs by default. It was
suppressed by -fno-warn-tabs.