[clang-doc] Avoid parsing undefined base classes

[juliehockett]

Summary

Don't try to parse base classes for declarations that are not definitions (segfaults, as there is no DefinitionData to access).

[lebedev.ri]

If it no longer crashes, i guess you can test for that?

Other than that, SG.

[juliehockett]

Adding test case

clang-tools-extra/clang-doc/Serialize.cpp

if (const auto *T = getDeclForType(P->getOriginalType())) {
       }
     }
     I.Params.emplace_back(P->getOriginalType().getAsString(),
                           P->getNameAsString());
   }
 }
 
 static void parseBases(RecordInfo &I, const CXXRecordDecl *D) {
+  // Don't parse bases if this isn't a definition.
+  if (!D->isThisDeclarationADefinition())
+    return;
   for (const CXXBaseSpecifier &B : D->bases()) {
     if (B.isVirtual())
       continue;
     if (const auto *P = getDeclForType(B.getType()))
       I.Parents.emplace_back(getUSRForDecl(P), P->getNameAsString(),
                              InfoType::IT_record);
     else
       I.Parents.emplace_back(B.getType().getAsString());