[CSBindings] Check literal coverage as new bindings/requirements are discovered

One more step towards incrementality of binding inference. Instead of
trying to determine literal protocol coverage during finalization
of the bindings, let's do that as soon as new bindings and/or literal
protocol requirements are discovered.

lib/Sema/ConstraintSystem.cpp

@@ -5330,11 +5330,7 @@ bool ConstraintSystem::isReadOnlyKeyPathComponent(
 TypeVarBindingProducer::TypeVarBindingProducer(
     ConstraintSystem::PotentialBindings &bindings)
     : BindingProducer(bindings.CS, bindings.TypeVar->getImpl().getLocator()),
-      TypeVar(bindings.TypeVar),
-      CanBeNil(llvm::any_of(bindings.Literals, [](const auto &literal) {
-        return literal.first->isSpecificProtocol(
-            KnownProtocolKind::ExpressibleByNilLiteral);
-      })) {
+      TypeVar(bindings.TypeVar), CanBeNil(bindings.canBeNil()) {
   if (bindings.isDirectHole()) {
     auto *locator = getLocator();
     // If this type variable is associated with a code completion token