TableGen: Add !dag function for construction

This allows constructing DAG nodes with programmatically determined
names, and can simplify constructing DAG nodes in other cases as
well.

Also, add documentation and some very simple tests for the already
existing !con.

Change-Id: Ida61cd82e99752548d7109ce8da34d29da56a5f7

Differential revision: https://reviews.llvm.org/D44110

llvm-svn: 327492