New builtin to get stack_frame object

[VoxSciurorum]

This pairs with a pull request to cheetah.

Add a C builtin __tapir_frame and IR intrinsic tapir_frame to get the current stack_frame object. These builtins return null if used outside of a Cilk region. Add a pointer to the stack_frame object as an argument when looking up a hyperobject view to avoid a TLS reference in the runtime.