The interior wrapping in Result<T, U> is a little unfortunate, but is
currently necessary because we end up with extra stack allocations
along the error-handling path in a do..catch that aren't there when
using untyped throws. We can simplify the implementation if we can
eliminate the extraneous stack allocation.
Fixes rdar://134973620.
Adds two new IRGen-level builtins (one for allocating, the other for deallocating), a stdlib shim function for enhanced stack-promotion heuristics, and the proposed public stdlib functions.
