[SPIRV] Handle unknown intrinsics (#166284)
This ports rather useful functionality that was already available in the
Translator, and was mostly implemented in the BE. Today, if we encounter
an unknown intrinsic, we pipe it through and hope for the best, which in
practice yields either obtuse ISEL errors, or potentially impossible to
use SPIR-V. With this change, if instructed via a newly added
`--spv-allow-unknown-intrinsics` flag, we emit allowed intrinsics as
calls to extern (import) functions. The test is also mostly lifted from
the Translator.
