[cxxmodules] Also load TreePlayer to fix TDataFrame tests.

It seems automatically loading TreePlayer when TDataFrame is used
without the appropriate include never worked. This was previously
fixed by just adding TreePlayer (which contains TDF) to the PCH.

As always, let's recreate this hack with modules to make restore
the old behavior with modules turned on.

core/metacling/src/TCling.cxx

@@ -1106,7 +1106,10 @@ static void LoadCoreModules(cling::Interpreter &interp)
    std::vector<std::string> optionalCoreModuleNames = {"stl", "libc"};
 
    // List of core modules we need to load.
-   std::vector<std::string> neededCoreModuleNames = {"Core", "RIO"};
+   // FIXME: TreePlayer is here because the rootmap loading doesn't work with
+   // TDataFrame. This should be gone once we no longer rely on rootmap files
+   // for loading a module.
+   std::vector<std::string> neededCoreModuleNames = {"Core", "RIO", "TreePlayer"};
    std::vector<std::string> missingCoreModuleNames;
 
    std::vector<clang::Module *> coreModules;