Support channel legalization strictness parameter in frontends

An upcoming change will move channel legalization strictness from being a global option to a per-channel option. For example, one channel could be configured as requiring mutually exclusive usages while another could be configured as allowing multiple accesses per tick.

Currently, the DSLX and XLSCC frontends do not set this option in the IR, so the default 'proven mutually exclusive' option will be used everywhere. Frontends should support ways of configuring channels in the IR they produce.