[TF:XLA] Failures from ComputationBuilder::IsConstant() and...

[TF:XLA] Failures from ComputationBuilder::IsConstant() and ComputationBuilder::ComputeConstant() should not be persistent.

The reason to note failures via first_error_ is so that most methods can defer failure reporting until Build() time. However this makes no sense for methods such as ComputeConstant() that return a Status already -- such methods can just return their errors directly without "polluting" other callers.
Change: 152129004