[XLA] Redesign: implement XlaBuilder::IsConstant,...

[XLA] Redesign: implement XlaBuilder::IsConstant, XlaBuidler::BuildConstantSubGraph, and Client::ComputeConstant(XlaComputation...).
- Since the builder no longer holds a client, we moved the ComputeConstant to the client side so that it can communicate with the service side. Now we add XlaBuilder::BuildConstantSubGraph, which is only responsible for building a subgraph that is compile-time constant.
- Before this change, every XlaBuilder has a unique id. Now since it also builds constant subgraph, we give every XlaComputation being built a global unique id, and uniquify instruction names when actually building the XlaComputation.

PiperOrigin-RevId: 192236997