Add diagnostics for returning a temporary, closes #381

[filipsajdak]

The current implementation of cppfront (65fcd0f) does not provide diagnostics for forward return temporary variables, e.g.:

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

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

Generates:

cpp2::i32 global {42}; 

[[nodiscard]] auto forward_return_literal() -> auto&&{
    return 1; // return reference to literal
}

[[nodiscard]] auto forward_return_rvalue() -> auto&&{
    return global + 1; // return reference to temporary variable
}

This change provides better diagnostics and handles most operators that might create a temporary variable:

• binary expression: /, <<, <=>, <, <=, >>, >=, >, +, -, ||, |, &&, *, %, &, ^, ==, !=,
• prefix expression: !,
• postfix expression: &, ~

All other operators that return references are accepted.

All regression tests pass. Closes #381

Need to be checked

The below cases still might not be supported:

• using operators that return a reference on local variables,
• calling function that returns a non-reference,

[JohelEGP]

I think https://en.cppreference.com/w/cpp/types/reference_converts_from_temporary could be used for the general case. See https://compiler-explorer.com/z/87b6aEEc8. But that's C++23 and incompletely supported:

[hsutter]

Thanks!