Allow concatenation of string literals at compile time

[vlstill]

Compiler-side support for p4lang/p4-spec#1297, that is support for compile-time string literal concatenation with ++.

The type checker and constant folding changes are rather straight forward. For the annotations, the constant folding will run automatically in the structured annotations (@foo[...]) but not in the unstructured (@foo(...)) unless they are parsed by the compiler explicitly. I've opted to explicitly allow @name, @deprecated and @noWarn to contain expressions, not just strings, which allows using ++ in them. A further catch was that BindTypeVariables can try to re-save expression's type into a type map, but if the expression is part of unstructured annotation (e.g. @deprecated), it was not type checked and therefore it is not in a type map. So I just prohibited BindTypeVariables from descending to unstructured annotations.

[vlstill]

@jonathan-dilorenzo, I would appreciate if you could try this works for your example too. If you annotations are structured it should work out of the box, if not you should be just able to parse them as expressions and then it should work too (similarly as I did for @name etc.).

Also I wonder if we should merge this alongside the spec change, or we can do this one first. The advantage of merging this first would be that it might allow a bit more testing in the backends that might be interested in using this.

[vlstill]

Since this was green-lit on the August LDWG (p4lang/p4-spec#1297 (comment)) I think we can merge this to main. If any issues arise when finalizing the spec, I will open another PR to remedy them.

In general I think any feature not in spec (e.g. loops) should be considered experimental and backends should use/acknowledge it only when they feel they can handle possible changes e.g. in syntax that may come before the feature is added to spec.