Depending on deployment target and platform, the function called to diagnose
reaching unavailable code could be `_diagnoseUnavailableCodeReached()` itself,
the back-deployment thunk, or the `@_alwaysEmitIntoClient` variant of the
function.
Resolves rdar://121344690
By default the compiler will now replace the bodies of unavailable functions
with stubs that call `_diagnoseUnavailableCodeReached()` instead.
Resolves rdar://116019744
When `-unavailable-decl-optimization=stub` is specified, insert a call to
`_diagnoseUnavailableCodeReached()` at the beginning of the function to cause
it to trap if executed at run time.
Part of rdar://107388493
