IRGen: Annotate runtime calls with willreturn attribute

`willreturn`
This function attribute indicates that a call of this function will
either exhibit undefined behavior or comes back and continues execution
at a point in the existing call stack that includes the current
invocation. Annotated functions may still raise an exception, i.a.,
`nounwind` is not implied. If an invocation of an annotated function does
not return control back to a point in the call stack, the behavior is
undefined.

I conservatively did not assume that the deinit is willreturn therefore
release like operations are not marked `willreturn`.

rdar://73574236

Author: aschwaighofer