[demangler] Remove StdQualifiedName

The StdQualifiedName node class is used for names exactly in the std
namespace.  It is not used for nested names that descend further --
those use a NestedName with NameType("std") as the scope.
Representing the compression scheme in the node graph is layer
breaking.  We can use the same structure for those exactly in std too,
and reduce code size a bit.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D118249