[SourceKit] Print custom attributes in interface-gen requests

Custom attributes were not printed because they are marked
'UserInaccesible'.

* Make CustomAttr 'RejectByParser' instead of 'UserInaccessible'
* Remove special treatment for Result Builder attributes
* Load implicit modules in module/header interface gen requests

rdar://79927502

tools/SourceKit/lib/SwiftLang/SwiftDocSupport.cpp

@@ -1086,6 +1086,12 @@ static bool reportModuleDocInfo(CompilerInvocation Invocation,
   ASTContext &Ctx = CI.getASTContext();
   registerIDERequestFunctions(Ctx.evaluator);
 
+  // Load implict imports so that Clang importer can use it.
+  for (auto unloadedImport :
+       CI.getMainModule()->getImplicitImportInfo().AdditionalUnloadedImports) {
+    (void)Ctx.getModule(unloadedImport.module.getModulePath());
+  }
+
   SourceTextInfo IFaceInfo;
   if (getModuleInterfaceInfo(Ctx, ModuleName, IFaceInfo))
     return true;