[#5331] Do not emit mismatch or overflow warnings for explicit casts

[kfcripps]

Addresses the issue of explicit casts being warned about, as discussed in #5331.

[kfcripps]

The last test case that I added shows an example where this PR does not warn about an implicit cast when it should be (before this PR, it was warned about). I am not sure if it would be better to add this warning to TypeInference (where the constant 72 is transformed into (bit<4>)72), or to propagate information to ConstantFolding about whether an IR::Cast was implicit or explicit so that it can decide to warn about implicit casts and not explicit casts.

[kfcripps]

or to propagate information to ConstantFolding about whether an IR::Cast was implicit or explicit so that it can decide to warn about implicit casts and not explicit casts.

I implemented this option for now.

[ChrisDodd]

If you make this optional bool implicit = false;, the two ctors will be autogenerated for you.

Also, the following comment is about destType, so this should be after that, not between the declaration of destType and the comment. Though given it is a /// comment it should be before the declaration, not after it (or have a <), so that Doxygen associates it with the member properly.

[kfcripps]

Ok the placement of that comment did seem strange to me.. I will move it above the destType declaration.