[cxxmodules] Undef `I` again (partly reverts 18473201222)
[cxxmodules] Undef `I` again (partly reverts 18473201222)

In commit 18473201222 we remove the code that undefs I
which breaks some tests since we now again export macros
from the STL module.

root-mirror/root,root-mirror/root,karies/root,root-mirror/root,olifre/root,karies/root,karies/root,olifre/root,root-mirror/root,olifre/root,olifre/root,root-mirror/root,olifre/root,olifre/root,olifre/root,karies/root,karies/root,karies/root,root-mirror/root,root-mirror/root,root-mirror/root,karies/root,karies/root,root-mirror/root,olifre/root,olifre/root,olifre/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,karies/root,karies/root