Add nodiscard attribute to Status and StatusOr

[PragmaTwice]

We make Status and StatusOr [[nodiscard]] in this PR, which means, you will get a compile warning if you do not handle the return value of a call to a function that returns Status or StatusOr, e.g.

Status DoSomething(int x);

DoSomething(0); // warning: cannot discard the return value

GET_OR_RET(DoSomething(0)); // fine if the current context is in a function that returns Status

if(auto s = DoSomething(0); !s.IsOK()) return s; // fine, same as above

if(auto s = DoSomething(0); !s.IsOK()) LOG(ERROR) << "got error: " << s.Msg(); // fine

auto s [[maybe_unused]] = DoSomething(0); // fine if you really want to discard the error (can explain in code review)

We will eventually make it a compiler error instead of a warning.