Inference timing: add a backtrace to head node (#38123)
This allows one to identify the caller of runtime-dispatched methods.
This can help detect inference problems and know where to fix them.
Only "head" nodes (entries into type inference) have backtraces.
We avoid doing any processing of the backtrace here, as instruction-pointer
lookup is extremely expensive and we don't even know if the information
will be used.
