[demangler] refactor SpecialSubKind

Code generating the special substitutions in std is a switch statement
with each case block containing the same conststruction template.  It
is more efficient to commonize that after the switch, having
determined which SubKind to create.  Also, let's sort the cases.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D118131