[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/CSSimplify.cpp

@@ -2288,6 +2288,7 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyConformsToConstraint(
       return SolutionKind::Solved;
     break;
   case ConstraintKind::ConformsTo:
+  case ConstraintKind::LiteralConformsTo:
     // Check whether this type conforms to the protocol.
     if (TC.conformsToProtocol(type, protocol, DC,
                               ConformanceCheckFlags::InExpression))
@@ -3467,6 +3468,7 @@ static TypeMatchKind getTypeMatchKind(ConstraintKind kind) {
     llvm_unreachable("Overload binding constraints don't involve type matches");
 
   case ConstraintKind::ConformsTo:
+  case ConstraintKind::LiteralConformsTo:
   case ConstraintKind::SelfObjectOfProtocol:
     llvm_unreachable("Conformance constraints don't involve type matches");
 
@@ -4126,6 +4128,7 @@ ConstraintSystem::simplifyConstraint(const Constraint &constraint) {
     return SolutionKind::Solved;
 
   case ConstraintKind::ConformsTo:
+  case ConstraintKind::LiteralConformsTo:
   case ConstraintKind::SelfObjectOfProtocol:
     return simplifyConformsToConstraint(
              constraint.getFirstType(),