Unique `Case` with multiple match behavior

[mkorbel1]

Motivation

When multiple cases match on a unique case statement, it currently causes an exception to be thrown. Perhaps this should be a configurable option allowing X's to be driven instead.

Desired solution

Always drive X when there's a violation of unique rules, and raise exception on it as well in a configurable way (default enabled) on a per-Case basis.

Alternatives considered

A global switch for this type of check could also be an option.

Additional details

SystemVerilog by default just prints a warning message (http://www.verilogpro.com/systemverilog-unique-priority/) which seems not great.