[cxxmodules] Fixed macro loading in LoadModule

In clang there are two records of what modules are visible: One
VisibleModuleSet is in the preprocessor, one is in the sema. In
the current code we only make every core module visible in sema,
but no in the preprocessor. This causes that we can load decls
from the core modules with that, but we can't load macros from
them as they are still invisible in the preprocessor.

This patch just also makes the module visible in the preprocessor.

core/metacling/src/TCling.cxx

@@ -1105,7 +1105,10 @@ static bool LoadModule(const std::string &ModuleName, cling::Interpreter &interp
    cling::Interpreter::PushTransactionRAII RAII(&interp);
    if (clang::Module *M = moduleMap.findModule(ModuleName)) {
       clang::IdentifierInfo *II = PP.getIdentifierInfo(M->Name);
-      return !CI.getSema().ActOnModuleImport(ValidLoc, ValidLoc, std::make_pair(II, ValidLoc)).isInvalid();
+      bool Result = !CI.getSema().ActOnModuleImport(ValidLoc, ValidLoc, std::make_pair(II, ValidLoc)).isInvalid();
+      // Also make the module visible in the preprocessor to export its macros.
+      PP.makeModuleVisible(M, ValidLoc);
+      return Result;
    }
    return false;
 }
@@ -1138,6 +1141,9 @@ static void LoadCoreModules(cling::Interpreter &interp)
 
    if (!LoadModule(moduleMap.findModule("RIO")->Name, interp))
       Error("TCling::LoadCodeModule", "Cannot load module RIO");
+
+   // Check that the gROOT macro was exported by any core module.
+   assert(interp.getMacro("gROOT") && "Couldn't load gROOT macro?");
 }
 
 static bool FileExists(const char *file)