[OM] Add field_locs array attribute for ClassFieldsOp locations

[leonardt]

This changes the ClassFieldsOp logic to use an inherent attribute to store an array mapping field to location by index. This avoids the issue where FusedLoc merges non-unique locations (preventing us from retrieving a fields location by mapping it to an index). It also avoids an issue where metadata is removed from the FusedLoc that was previously being used to store this information.

[mikeurbach]

Generally looks good, just a couple minor things from my end. I guess one question is if this should go on the ClassOp (like we do for HWModuleOp) or on the ClassFieldsOp. I don't have any particularly strong reason to go one way or another. Keeping it on ClassFieldsOp like this PR does seems reasonable, since existing code expects to get field locations from the ClassFieldsOp.

[mikeurbach]

Also please update the description / commit message (really, the commit message when this merges), to explain why we want to make this change.

[leonardt]

Re: putting this in ClassOp, since we may need to implement a custom parser/printer for this, there could be motivation to just hook onto the existing parser/printer for ClassOp, but I think since it's information associated with the fields op it does make sense to store it with the op.

[mikeurbach]

A couple more minor comments. I think this would be fine to go in now, but a couple minor niceties that could help.

[uenoku]

LG