[REG2.066a] Issue 13141 - array cast from string[] to immutable(char[][]) is not supported at compile time

https://issues.dlang.org/show_bug.cgi?id=13141
Prevent unnecessary `CastExp` addition for unique conversion on pure function return.

By this change, `Expression::implicitCastTo` will be able to handle the case.

```cpp
        void visit(Expression *e)
        {
            //printf("Expression::implicitCastTo(%s of type %s) => %s\n", e->toChars(), e->type->toChars(), t->toChars());

            MATCH match = e->implicitConvTo(t);
            if (match)
            {
                if (match == MATCHconst &&   // hit!
                    (e->type->constConv(t) ||
                     !e->isLvalue() && e->type->immutableOf()->equals(t->immutableOf())))
                {
                    /* Do not emit CastExp for const conversions and
                     * unique conversions on rvalue.
                     */
                    result = e->copy();
                    result->type = t;
                    return;
                }
                result = e->castTo(sc, t);
                return;
            }
```