[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