[MachineOutliner] Disable outlining from noreturn functions

Outlining from noreturn functions doesn't do the correct thing right now. The
outliner should respect that the caller is marked noreturn. In the event that
we have a noreturn function, and the outlined code is in tail position, the
outliner will not see that the outlined function should be tail called. As a
result, you end up with a regular call containing a return.

Fixing this requires that we check that all candidates live inside noreturn
functions. So, for the sake of correctness, don't outline from noreturn
functions right now.

Add machine-outliner-noreturn.mir to test this.

llvm-svn: 373791