Since f16ddcee / D876, `ghc-stage1`
supports a subset of `-XTemplateHaskell`, but since we need Cabal to be
able detect (so `.cabal` files can be specified accordingly, see also
GHC #11102 which omits `TemplateHaskell` from `--supported-extensions`)
whether GHC provides full or only partial `-XTemplateHaskell` support,
the proper way to accomplish this is to split off the
quotation/non-splicing `TemplateHaskell` feature-subset into a new
language pragma `TemplateHaskellQuotes`.

Moreover, `-XTemplateHaskellQuotes` is considered safe under SafeHaskell

This addresses #11121

Reviewers: goldfire, ezyang, dterei, austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1511

GHC Trac Issues: #11121