Previously, any Template Haskell code that was spliced would
be marked as `Generated`, which would completely suppress pattern-
match coverage warnings for it, which several folks found confusing.
Indeed, Template Haskell-spliced code is "source" code in some sense,
as users specifically request that it be put into their program, so
changing its designation to `FromSource` makes sense from that
perspective.

Test Plan: make test TEST=T14838

Reviewers: goldfire, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #14838

Differential Revision: https://phabricator.haskell.org/D4440