Sema: use the underlying type when available instead of the structural type request

The underlying type may have been set on a validated type alias or when deserialized.

Author: xymus

lib/AST/GenericSignatureBuilder.cpp

@@ -3806,8 +3806,12 @@ PotentialArchetype *GenericSignatureBuilder::realizePotentialArchetype(
 }
 
 static Type getStructuralType(TypeDecl *typeDecl) {
-  if (auto typealias = dyn_cast<TypeAliasDecl>(typeDecl))
+  if (auto typealias = dyn_cast<TypeAliasDecl>(typeDecl)) {
+    if (auto resolved = typealias->getUnderlyingTypeLoc().getType())
+      return resolved;
+
     return typealias->getStructuralType();
+  }
 
   return typeDecl->getDeclaredInterfaceType();
 }