[XLA:CPU] Migrate aot/runtine.{h,cc} to xla_compiled_cpu_function.{h,cc}

As a follow-on cleanup for cl/206980796 ("Overhaul XLA:CPU's calling
convention.") I want to introduce a BufferInfo class that encapsulates whether a
buffer is a constant, an entry parameter or a temp without using the fragile
"size < 0" scheme I have today.  To do this efficiently I need a place to put
the BufferInfo class that will be visible to MallocContiguousBuffers.  Instead
of creating (what seemed to me) an odd layering with BufferInfo in aot/runtime.h
I decided to pull in the runtime into xla_compiled_cpu_function since that's the
only user.

PiperOrigin-RevId: 207333245