Clean up the interface to Type::subst(), NFC.

Replace the 'ignoreMissing' boolean flag with a new option set type,
SubstOptions, which is easier to extend. It is not an OptionSet<>
because a follow-on commit will introduce a non-trivial option that
will require more storage.

Also eliminate the LazyResolver parameter, which is no longer
needed. Eliminate the silly TypeChecker::substType(), whose only
purpose was to provide the resolver.

Swift SVN r27656

lib/AST/Module.cpp

@@ -691,9 +691,10 @@ ArrayRef<Substitution> BoundGenericType::getSubstitutions(
   index = 0;
   for (auto archetype : allArchetypes) {
     // Substitute into the type.
-    auto type = Type(archetype).subst(module, substitutions,
-                                      /*ignoreMissing=*/hasTypeVariables,
-                                      resolver);
+    SubstOptions options;
+    if (hasTypeVariables)
+      options |= SubstOptions::IgnoreMissing;
+    auto type = Type(archetype).subst(module, substitutions, options);
     if (!type)
       type = ErrorType::get(module->getASTContext());