This replaces some occurences of `-f(no-)warn` with the new `-W`-aliases
introduced via 2206fa8c / #11218, in cases which are guaranteed
to be invoked with recent enough GHC (i.e. the stage1+ GHC).

After this commit, mostly the compiler and the testsuite remain using
`-f(wo-)warn...` because the compiler needs to be bootstrappable with
older GHCs, while for the testsuite it's convenient to be able to quickly
compare the behavior to older GHCs (which may not support the new flags yet).

The compiler-part can be updated to use the new flags once GHC 8.3 development
starts.

Reviewed By: quchen

Differential Revision: https://phabricator.haskell.org/D1637