White space
Introduce Lookup::Named overload taking a StringRef
Introduce shortcut in LookupHelper::findScope.

In simple case not involving templates or template instantiation, avoid creating a memory
buffer and calling the Parser.
Avoid unnecessary ClassInfo creations and string copy.

With CINT those creation were necessary to avoid spurrious ignore message about non-existing scope,
those messages are not issued by the cling routines.  Also the string copy is now unnecesary (since
the string is now unmodified).