[cxxmodules] Module cache should point to the library dir.

Author: vgvassilev

core/metacling/src/TCling.cxx

@@ -1208,9 +1208,12 @@ TCling::TCling(const char *name, const char *title)
    // to rootcling to set this flag depending on whether it wants to produce
    // C++ modules.
    TString vfsArg;
+   TString moduleCachePathArg;
    if (useCxxModules && !fromRootCling) {
       // We only set this flag, rest is done by the CIFactory.
       interpArgs.push_back("-fmodules");
+      moduleCachePathArg = "-fmodules-cache-path=" + TROOT::GetLibDir();
+      interpArgs.push_back(moduleCachePathArg.Data());
       // We should never build modules during runtime, so let's enable the
       // module build remarks from clang to make it easier to spot when we do
       // this by accident.