WIP: Add constexpr if

[rw1nkler]

This PR adds support for the constexpr if expression.

Information about the const modifier is stored in the Conditional class and used later during type checking. For constexpr if, separate type variables are assigned to each branch of the condition. At this stage, the result is not directly bound to them. During the TypeInfo generation, the constexpr condition is evaluated, and one branch of the if is selected. Its type is unified, and the entire if is annotated with that type. The bytecode emitter and IR converter have been updated to inline the selected branch, skipping the unchosen one.

Proper tests for this feature are still needed and will be added soon.

One issue not yet handled correctly in this PR is the conditional spawning of procs, for which an example has been added that shows the problem. Currently, all spawns affect the type, which should not happen as only the selected branch of constexpr if should be considered. One possible approach is to enable constexpr evaluator in the PopulateInferenceTableVisitor and skip traversing the branch that was not taken. However, the PopulateInferenceTableVisitor does not have access to the parametric environment, and the condition relying on the parametrics seems to be the most imprtant use-case for using constexpr if when spawning procs.

[richmckeever]

I think this would be more in-model if we would define a new type of internal type annotation, like ConstConditionalTypeAnnotation, and set an instance of that on the Conditional node in the const case. The ConstConditionalTypeAnnotation could have the const test Expr as a member, and a TVTA for each possible branch.

TypeAnnotationResolver would then perform the resolution logic that you currently have inside GenerateTypeInfo (in a manner similar to MemberTypeAnnotation, ElementTypeAnnotation, etc.).

Aside from that this all looks reasonable.

[richmckeever]

Usually we prefer to use positive conditions in a C++ if statement, even if it leads to the less common branch being first (so flip the 2 branches here).