Recommit r231221: "Devirtualize ~parser<T> by making it protected in base...

Recommit r231221: "Devirtualize ~parser<T> by making it protected in base classes and making derived classes final"

Reverted in r231254 due to a self-hosting crash of Clang (see Clang
PR22793). Workaround the crash by using {} instead of = default to
define a dtor.

llvm-svn: 231274