If a linkonce_odr dtor/ctor is identical to another one, just rauw.

Unlike an alias a rauw is always safe, so we don't need to avoid this
optimization when the replacement is not know to be available in every TU.
Remove an incorrect optimization inside Clang's IRGen. Its check to determine
whether we can safely lower a conditional operator to select was insufficient.
I've left a large comment in place to explaining the sort of problems that this
transform can encounter in clang in the hopes of discouraging others from
reimplementing it wrongly again in the future. (The test should also help with
that, but it's easy to work around any single test I might add and think that
your particular implementation doesn't miscompile any code.)