[LTL] Make PastOp clock operand mandatory

[TaoBi22]

The implicit clocking of ltl.past complicates lowering as it leaves you having to infer the op's clocking behaviour from context. Since as of #10370 we no longer create any PastOps without an explicit clock, this makes the clock operand mandatory avoid implicit clocking behaviour. This requires a few changes to the FIRRTL intrinsic too - please let me know if that's likely to cause any problems.

Note that this leaves assume-first-clock in LTLToCore redundant - I'll remove it in a follow-up since it seems that the other implicitly clocked ops are also shifting to an explicit model too (e.g. #9392 and follow-ups)

[fabianschuiki]

Results of circt-tests run for 34e393b compared to results for 5ae6aef: no change to test results.

[TaoBi22]

Marking as draft until Chisel's been updated to not emit implicitly clocked past intrinsics.

[TaoBi22]

Marking as draft until Chisel's been updated to not emit implicitly clocked past intrinsics.

Fixed in Chisel as of cb507359fad35be779d28acc2ad910be0bb21172