While it is still technically possible to test stage 0, it is not part of any
of the main testing rules and maintaining xfail-stage0 is a chore. Nobody
should worry about how tests fare in stage0.
An expression like:
foo(1, fail, 2)
was failing to parse, because the parser was interpreting the comma
as the start of an expression that was an argument to fail, rather
than recognizing that the fail here has no arguments
Fixed this by using can_begin_expr to determine whether the next
token after a fail token suggests that this is a nullary fail or a
unary fail.
In addition, when translating calls, check before translating each
argument that the block still isn't terminated. This has the effect
that if an argument list includes fail, the back-end won't keep trying
to generate code for successive arguments and trip the !*terminated
assertion.
