Initial XLA support for TF eager. This is prerequisite for TF compiler's XLA support.

This CL adds XLA support for the following TFE_Op's:

1. A TF op such as MatMul, with full support of constant and resource params.

2. A TF_Function as TFE_Op, where the function must have no constant and
resource params. Removing this restriction requires more discussion and will be
deferred to a later time.

PiperOrigin-RevId: 184877345