spirv: Make vtn_fail and friends trap in debug mode

The SPIR-V parser catches its failures and failed assertions, clean up
and return a failure to the caller.  This is a good default behavior, but
when debugging sometimes is useful to hook the debugger right in the
failed assertion.

Reviewed-by: Jason Ekstrand <jason.ekstrand@collabora.com>
Part-of: <mesa/mesa!19663>