ClangImporter: Fix up importSubscript() to not depend on import order

Once lazy loading supports mirrored protocol members, we can end up
calling importSubscript() for a setter method before we've mirrored
the getter. In this case, we call findCounterpart() with the setter,
which finds the imported getter from the ProtocolDecl itself.

Stop processing the potential subscript in this case, since when we
later mirror the protocol member, we'll go and build it again.

This should be NFC without the next change I'm working on.

Author: slavapestov

lib/ClangImporter/ImportDecl.cpp

@@ -6585,6 +6585,13 @@ SwiftDeclConverter::importSubscript(Decl *decl,
     if (!counterpart)
       return nullptr;
 
+    // If we're looking at a class but the getter was found in a protocol,
+    // we're going to build the subscript later when we mirror the protocol
+    // member. Bail out here, otherwise we'll build it twice.
+    if (interface &&
+        isa<clang::ObjCProtocolDecl>(counterpart->getDeclContext()))
+      return nullptr;
+
     return cast_or_null<FuncDecl>(
         Impl.importDecl(counterpart, getActiveSwiftVersion()));
   };