Support for standard type union discriminant with constant value

[bartekn]

Union.resolved_case doesn't support unions with standard type discriminant and constant (const) values, ex:

enum StellarValueType
{
    STELLAR_VALUE_BASIC = 0,
    STELLAR_VALUE_SIGNED = 1
};

union switch (int v)
{
case STELLAR_VALUE_BASIC:
    void;
    ...

To fix this, we are searching for identifier in namespace's enum constants if discriminant type has not been found.

EDIT, realized that it probably doesn't work for JS and Java too:

• Update JS
• Update Java

[nullstyle]

I'm cool with this generalized approach to solving the problem, but we should make sure to update all of the language generators and confirm they work before merging.

[bartekn]

@nullstyle done. Unfortunately Java and JS solution is tricky.

In Java, the type of the value we use switch on is Integer and the types must match in cases. However, StellarValueType.STELLAR_VALUE_BASIC is not Integer and StellarValueType.STELLAR_VALUE_BASIC.getValue() is not constant (getting "Constant expression required" error).

In JS, I was getting "ChildUnion._switchOn.fromName is not a function" error because _switchOn on the union is equal xdr.int().

In both cases I ended up with rendering actual numeric values of constants (0 for STELLAR_VALUE_BASIC and 1 for STELLAR_VALUE_SIGNED). This is not ideal but is a working solution.

If you have a better idea how to solve this don't hesitate and change my code.