Fix regression incorrectly advertising TH support

--supported-languages must only advertise language extensions which are supported by the compiler in order for tooling such as Cabal relying on this signalling not to behave incorrectly.

NB: This needs to be backported to the ghc-8.6 branch for the upcoming GHC 8.6.4 release as well as the ghc-8.8 branch; but I couldn't figure out a way to target those branches via MRs

Fixes #16331 (closed)