ORC_RT_BUILTIN_UNREACHABLE is an optimizer hint that a location is
unreachable, wrapping __builtin_unreachable / __assume(false). Executing
it is undefined behavior.
ORC_RT_UNREACHABLE(MSG) marks a point the program must never reach. It
aborts with MSG in +Asserts builds; otherwise it lowers to
ORC_RT_BUILTIN_UNREACHABLE.
Adds a CompilerTest unit test covering both the returning-function
compile path and, in +Asserts builds only, the abort behavior via a
death test.