`ghc --supported-extension` shall not list `TemplateHaskell` when unsupported

Currently GHC advertises TemplateHaskell even though it's statically known to fail:

$ ghc --supported-extensions | grep Template
TemplateHaskell
NoTemplateHaskell

# `NoTemplateHaskell` works:
$ ghc -XNoTemplateHaskell -c hello.hs

# `TemplateHaskell` however fails even though advertised:
$ ghc -XTemplateHaskell -c hello.hs
ghc: on the commandline: Template Haskell requires GHC with interpreter support
    Perhaps you are using a stage-1 compiler?
Usage: For basic information, try the `--help' option.

Having properly advertised extensions is particularly useful in combination with Cabal#2644 allowing the solver to toggle flags depending on the availability of -XTemplateHaskell.

For instance, cross-compilers and/or unregisterised GHC builds often don't have TemplateHaskell support. Having support for toggling flags based on availability of -XTemplateHaskell allows cabal to support such environments with less manual intervention.

(See also https://github.com/haskell/cabal/pull/2930 for pre-GHC8 emulation of this feature)

Trac metadata

Trac field	Value
Version	7.10.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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