Make Objective-C++ match Objective-C's behavior on implicit ivar access when `self` is shadowed

[mwyman]

Summary

When compiling Objective-C++, name lookup uses the C++ lookup behavior that skips the filtering of non-implicit-self references, and will use a shadowed name that matches the type. As a result implicit ivar access behavior differs between Objective-C and Objective-C++, which can impact local access but also block captures.

This addresses the issue #56193 as reported on Github.

Unfortunately it appears the behavior has differed for 11 years (since the original fix for ObjC was applied in 2011).

[gribozavr2]

Please add a test.

[mwyman]

Added codegen test.

[mwyman]

Unit test passes both pre- and post- this change for -x objective-c, fails pre-change with -x objective-c++ and passes post-change.

[mwyman]

Change to use a unit test on the semantic lookup code, rather than a codegen test; I realized the name lookup behavior could be more directly checked by inspecting the internal references in the generated AST to ensure the found "self" is the implicit parameter "self".

clang/lib/Sema/SemaLookup.cpp

for (; I != IEnd && S->isDeclScope(*I); ++I) {
             LeftStartingScope = true;
 
           // If we found something outside of our starting scope that
           // does not have linkage, skip it.
           if (LeftStartingScope && !((*I)->hasLinkage())) {
             R.setShadowed();
             continue;
           }
+        } else if (NameKind == LookupObjCImplicitSelfParam &&
+                   !isa<ImplicitParamDecl>(*I)) {
+          continue;
         } else {
           // We found something in this scope, we should not look at the
           // namespace scope
           SearchNamespaceScope = false;
         }
         R.addDecl(ND);
       }
     }