[ASTImporter] Add Visitor for TypedefNameDecl's

[vabridgers]

Summary

We found a case where Typedef Name Declarations were not being added
correctly when importing builtin types. This exposed the need for a
TypedefNameDecl visitor so these types can be added by RecordDecl and
fields.

This code is covered by the ASTImporterTest cases that use the implicit
struct __NSConstantString_tag definitions.

Thanks to @martong for the debugging assist!

Depends on D83970.

[vabridgers]

Fix lint pre-merge check

[vabridgers]

I discussed with Gabor. We don't feel comfortable landing this without a covering negative test case, so I'll work on that a little more, see what I can come up with. Thanks Gabor!

[vabridgers]

Adding negative test case that exposes the original problem.

[martong]

LGTM! Thanks for the test!

clang/lib/AST/ASTImporterLookupTable.cpp

 
 namespace clang {
 
 namespace {
 
 struct Builder : RecursiveASTVisitor<Builder> {
   ASTImporterLookupTable &LT;
   Builder(ASTImporterLookupTable &LT) : LT(LT) {}
+
+  bool VisitTypedefNameDecl(TypedefNameDecl *D) {
+    QualType Ty = D->getUnderlyingType();
+    Ty = Ty.getCanonicalType();
+    if (const auto *RTy = dyn_cast<RecordType>(Ty)) {
+      LT.add(RTy->getAsRecordDecl());
+      // iterate over the field decls, adding them
+      for (auto it : RTy->getAsRecordDecl()->fields()) {
+        LT.add(it);
+      }
+    }
+    return true;
+  }
+
   bool VisitNamedDecl(NamedDecl *D) {
     LT.add(D);
     return true;
   }
   // In most cases the FriendDecl contains the declaration of the befriended
   // class as a child node, so it is discovered during the recursive
   // visitation. However, there are cases when the befriended class is not a
   // child, thus it must be fetched explicitly from the FriendDecl, and only