Remove old concepts parsing code

Summary:
This is so we can implement concepts per P0734R0. Relevant failing test
cases are disabled.

Reviewers: hubert.reinterpretcast, rsmith, saar.raz, nwilson

Reviewed By: saar.raz

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D40380

Patch by Changyu Li!

llvm-svn: 319992