Properly handled (partial) desugaring of 'const NS::Something_t'

TClassEdit::ResolveTypedef was not properly handling the leading const
if there is a namespace in the name (and no template).

Transform::GetPartiallyDesugaredType was not keeping the 'const' in case
the target of the typedef was in the global scope (for example being a
fundamental type).

This fixes ROOT-5576
Add comment clarifying ROOT-4602