[XLA] Rename Interpreter{Executor,Platform} -> XlaInterpreter{Executor,Platform}.

These types live inside StreamExecutor's namespace, but they are
specific to XLA.  Therefore they either shouldn't live in SE's namespace
or should have "XLA" in the name.

Moving them out of SE's namespace is ugly, because almost every type
used inside of these headers then needs to be qualified.  So name-change
it is.

This patch was generated by a mechanical find/replace.

PiperOrigin-RevId: 192724238