[TableGen] Support named arguments

We provide a way to specify arguments in the form of `name=value`
so that we don't have to specify all optional arguments before the
one we'd like to change. Required arguments can alse be specified
in this way.

Note that the argument can only be specified once regardless of
the way (named or positional) to specify and positional arguments
should be put before named arguments.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D152998