Preserve parens in TH

Template Haskell supports parentheses in its AST. So does GHC. Yet GHC casually discards parentheses when desugaring TH quotes. We should preserve them.

I was originally thinking that this could get rid of Note [Adding parens for splices] in Convert, but that processing would still be necessary if we are converting TH AST that did not come from a quote. Actually, it gets worse with this change, because we don't want to add redundant parens.

Still, I think preserving parens is the right thing to do here. It also makes it possible for a client to give special meaning to, say, nested parens: Perhaps process [| f x |] and process [| f ((x)) |] will behave differently. (Idiom brackets, anyone?)

Trac metadata

Trac field	Value
Version	8.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
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