Add XlaCompiledFunction, a lightweight API for calling XLA computations that are

compiled down to functions. The API is based on a generic form of the original
AOT auto-generated header.

For AOT (tfcompile), this API has been slotted into the auto-generated header.

For JIT, a new XlaCompiledFunctionJit class has been added, which compiles a
tensorflow::GraphDef and allows the user to create XlaCompiledFunction objects.

XlaCompiledFunction contains optional metadata; mappings from arg/result names
to their index, and the program shape. This data is always available via JIT,
but only provided via AOT if the tfcompile --gen_name_to_index and
--gen_program_shape flags are set. We don't enable by default for AOT to keep
binary sizes smaller; the ProgramShape proto pulls in lots of code, and may also
be large.

PiperOrigin-RevId: 170811579