[llvm] Revert r231274: "Devirtualize ~parser<T> by making it protected in base...

[llvm] Revert r231274: "Devirtualize ~parser<T> by making it protected in base classes and making derived classes final"

Summary: This patch was previously applied in r231221, and reverted in
r231254 because it broke self-hosting.  It was subsequently fixed and
reapplied in r231274.  Unfortunately, making the `parser<T>` classes
final prevents inheritance which makes it impossible to implement
custom parsers.

Reverting r231221 restores the ability to customize parsers.

Reviewers: dblaikie

Reviewed By: dblaikie

Subscribers: craig.topper, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60955

llvm-svn: 359902