ICE on comptime IIFE using both argument & return type inference

[bb010g]

(Tested with Godbolt's Zig 0.6.0 & trunk builds.)

comptime {
    const hold_the_ice: bool = true;
    if (hold_the_ice) {
        @compileLog(struct { pub fn go() var { return "properly errors"; } }.go());
    } else {
        @compileLog("ICE ICE baby");
        @compileLog(struct { pub fn go(val: var) var { return val; } }.go("woah"));
        @compileLog("ICE melted");
    }
}

Output:

<source>:4:34: error: TODO implement inferred return types https://github.com/ziglang/zig/issues/447
        @compileLog(struct { pub fn go() var { return "properly errors"; } }.go());
                                 ^
<source>:4:77: note: referenced here
        @compileLog(struct { pub fn go() var { return "properly errors"; } }.go());
                                                                            ^
Compiler returned: 1

Output with const hold_the_ice: bool = false;:

| *"ICE ICE baby"
Compiler returned: 255

See also: #447.