[BUG] Forward returning literals and rvalues not rejected.

[leejy12]

This Cpp2 code

global: i32 = 42;

forward_return_literal: () -> forward _ = {
    return 1;
}

forward_return_rvalue: () -> forward _ = {
    return global + 1;
}

compiles to

cpp2::i32 global {42}; 

[[nodiscard]] auto forward_return_literal() -> auto&&{
    return 1; 
}

[[nodiscard]] auto forward_return_rvalue() -> auto&&{
    return global + 1; 
}

which is problematic Cpp1. We should err when forward returning literals and rvalues.

[filipsajdak]

This is about returning temporary variables. I think I have a fix. I will test it and push it.

[filipsajdak]

I have pushed the fix: #384