We currently require that all referenced globals have an explicit
declaration or definition in the IR. For intrinsics, this requirement is
redundant, because they cannot be called indirectly (including "direct"
calls with mismatched function type). The function type used in the call
directly determines the function type of the intrinsic declaration.
Relax this requirement, and implicitly declare any intrinsics that do
not have an explicit declaration. This will remove a common annoyance
when writing tests and alive2 proofs.
(I also plan to introduce a mode where declarations for all missing
symbols will be automatically added, to make working with incomplete IR
easier -- but that will be behind a default-disabled flag.)
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