If a sugared type desugars to a substitutable type, we would
return the replacement type without the sugar. I think in
practice this meant that ParenType would be lost sometimes.
Preserving this correctly is required for an upcoming CSDiag
change.
Note that there's a minor source-breaking change with enum
case constructors here. I've filed <rdar://problem/27261929>
to track sorting it out in Swift 3 mode.
Also an upcoming patch fixes another related issue and adds more
tests for case constructors.
We had a few places that were performing ad hoc variants of
ConstraintSystem::getFixedTypeRecursive(); simplify it's interface so
we can use it everywhere consistently. Fixes rdar://problem/27261929.
