[core] Flag explicit tmplt spec as autoloadable:

With llvm9, the external AST source will only be asked to provide the
definition of an explicit specialization if the forward declaration of
the explicit specialization is flagged as hasExternalLexicalStorage().

In roottest/root/meta/runtemplateAutoload*, the external AST source
is the autoloading; the fwd decl comes from the rootmap file: the
specialization "resides" in a library that needs to be loaded, instead
of instantiating the template from the default definition.

Walk the declarations parsed by the rootmap reader, and flag all
explicit specializations as "has more info". For that to work, explicit
specializations must show up in the declarations section of the rootmap
files.