[TableGen] Correct comments for end of namespace. NFC

Summary:
Update end-of-namespace comments generated by
tablegen emitters to fulfill the rules setup by
clang-tidy's llvm-namespace-comment checker.

Fixed a few end-of-namespace comments in the
tablegen source code as well.

Reviewers: craig.topper

Reviewed By: craig.topper

Subscribers: craig.topper, stoklund, dschuff, sbc100, jgravelle-google, aheejin, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66396

llvm-svn: 369865