cgen: fix auto deref for interface variable

[felipensp]

Fix #19834

🤖 Generated by Copilot at 76cc728

Fix bugs in casting interface variables to strings in cgen.v, if.v, and infix.v. Add a new field and a helper function to handle interface variable dereferencing in different contexts.

🤖 Generated by Copilot at 76cc728

• Add a new function is_interface_var to simplify checking interface variables (link)
• Fix casting interface variables to strings in if conditions and infix expressions by setting and restoring the inside_casting_to_str field of the Gen struct (link, link)
• Use the is_interface_var function to simplify and fix the condition for writing a * before the object name in the gen_call_expr method in cgen.v (link)

[spytheman]

Good work.

[spytheman]

@felipensp can you please also take care of that:

./v -cstrict -cc clang x_test.v, where x_test.v is:

interface Param {}

fn test_main() {
        param := Param(false)
        match param {
                bool {
                        if param {
                                dump(param) // switching the dump and the println makes it compile
                                println(param)
                        }
                }
                else {}
        }
}

[felipensp]

@felipensp can you please also take care of that:

Done. Thanks.