[TableGen] Add !setdagarg and !setdagname

- This patch proposes to add `!setdagarg` and `!setdagname` bang
  operators to produce a new DAG node after replacing the specified
  argument value/name from the given input DAG node. E.g.,
  `!setdagarg((foo 1, 2), 0, "x")` produces `(foo "x", 2)` and
  `!setdagname((foo 1:$a, 2:$b), 1, "c")` produces `(foo 1:$a, 2:$c)`.

Reviewed By: simon_tatham

Differential Revision: https://reviews.llvm.org/D151842