[HW] Move NonLocalAnchor from FIRRTL dialect to HWDialect

[prithayan]

The NonLocalAnchor can be used to specify the instance path for uniquely identifying any operation globally. It is used to attach nonlocal annotations in the FIRRTL dialect. Currently it is dropped in the LowerToHW pass.

Move the NonLocalAnchor from FIRRTL dialect to HW dialect, so that we can represent the hierarchical paths in the HW dialect also. This enables the metadata emission to use it to specify the hierarchical paths. In a followup PR, we can enable ExportVerilog to lower this op to a corresponding instance path when used for macro substitution in a VerbatimOp.

[lattner]

Could we get a design doc or expansion of a rationale doc to explain what this is and what it is used for?

[prithayan]

Updated the rationale docs with this change.

[teqdruid]

@prithayan Is this PR still relevant? If not, please close it.

[darthscsi]

I think subsumed by later work, closing.