[flang] Fix bogus error on aliased derived type (#69959)

When a derived type definition is visible by two or more names in the
same scope due to USE renaming, any generic ASSIGNMENT(=) or OPERATOR()
bindings in the type will produce incorrect error messages about
indistinguishable specific procedures. This is due to the use of a
std::vector<> to hold a sequence of symbols and some derived information
for the specific procedures of the generic name. Change to a std::map<>
indexed by the ultimate symbol of each specific so that duplicates
cannot arise.