[AST] Adjust getBridgedToObjC to produce only valid witness types

`getBridgedToObjC` was allowed to produce a dependent member type with
invalid base (`<<error type>>`) if Objective-C import is broken,
which results in a crash during member lookup on that type by the
 constraint solver.

Resolves: rdar://104354485

Author: xedin

lib/AST/ASTContext.cpp

@@ -5464,7 +5464,10 @@ Type ASTContext::getBridgedToObjC(const DeclContext *dc, Type type,
       *bridgedValueType = type;
 
     // Find the Objective-C class type we bridge to.
-    return conformance.getTypeWitnessByName(type, Id_ObjectiveCType);
+    Type witnessTy = conformance.getTypeWitnessByName(type, Id_ObjectiveCType);
+    // If Objective-C import is broken, witness type would be a dependent member
+    // with `<<error type>>` base.
+    return (witnessTy && !witnessTy->hasError()) ? witnessTy : Type();
   }
 
   // Do we conform to Error?