[TableGen] Update editor modes for new keywords.

Summary:
D71407 and D71474 added new keywords to the Tablegen language:
`defvar`, `if`, `then` and `else`. This commit updates the various
editor modes to highlight them appropriately.

Some of the modes also didn't include `defset`, so I've added that too
while I was there.

Reviewers: MaskRay, lebedev.ri, plotfi

Reviewed By: lebedev.ri

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72693