fail!() used to require owned strings but can handle static strings
now. Also, it can pass its arguments to fmt!() on its own, no need for
the caller to call fmt!() itself.
This was at least partially responsible for Issue 777.
The only solution I can think of is for trans to just not generate
code for a comparison if one or both sides has type _|_. Since
that means evaluating that subexpression diverges, it should be ok
to never do the comparison. Actually generating code for the
comparison would trip an LLVM assertion failure.
