[cxxmodules] Teach ACLiC to build modules.

ACLiC now synthesizes a modulemap with a suffix _ACLiC_dict.modulemap. The file
contains the source file to be compiled and the corresponding library.

The modulemap is then passed to rootcling via -fmodule-map-file= flag to avoid
naming clashes with possibly existing other modulemap files.

This patch teaches cling to work with the -fmodule-map-file= flag.

ACLiC supports automatic inclusion of Rtypes.h (making ClassDef macro
available). Modules are built in isolation and are resilient to #include
of Rtypes.h at rootcling startup time. We make module Core (containing Rtypes.h)
visible via a newly implemented callback.