[Type checker] Eliminate the 'literalConformanceProto' state on type variables.

The 'literalConformanceProto' field of
TypeVariableType::Implementation didn't take into account equivalence
classes of type variables. Eliminate it, and either look at the actual
expressions (for optimizing constraints during constraint generation)
or the actual constraints on a given type variable (for determining
whether to include optionals in the set of potential type variable
bindings).

(cherry picked from commit 6bdd9cfae5)

lib/Sema/CSDiag.cpp

@@ -2920,7 +2920,8 @@ bool FailureDiagnosis::diagnoseGeneralConversionFailure(Constraint *constraint){
   // If simplification has turned this into the same types, then this isn't the
   // broken constraint that we're looking for.
   if (fromType->isEqual(toType) &&
-      constraint->getKind() != ConstraintKind::ConformsTo)
+      constraint->getKind() != ConstraintKind::ConformsTo &&
+      constraint->getKind() != ConstraintKind::LiteralConformsTo)
     return false;