AK: Don't implicitly convert Optional<T&> to Optional<T>

C++ will jovially select the implicit conversion operator, even if it's
complete bogus, such as for unknown-size types or non-destructible
types. Therefore, all such conversions (which incur a copy) must
(unfortunately) be explicit so that non-copyable types continue to work.

NOTE: We make an exception for trivially copyable types, since they
are, well, trivially copyable.

Co-authored-by: kleines Filmröllchen <filmroellchen@serenityos.org>

Libraries/LibUnicode/CurrencyCode.cpp

@@ -197,7 +197,7 @@ static auto const& ensure_currency_codes()
     return currency_codes;
 }
 
-Optional<CurrencyCode> get_currency_code(StringView currency)
+Optional<CurrencyCode const&> get_currency_code(StringView currency)
 {
     static auto const& currency_codes = ensure_currency_codes();
     return currency_codes.get(currency);