On a few compilers (clang 11/12 for example [1]), the following does not
result in a copy elision, and since `Error`'s copy dtor is elided,
results in a compile error:
```
Expect<Something> foobar() {
...
if (Error E = aCallReturningError())
return E;
...
}
```
Moving `E` would, conversely, result in the pessimizing-move warning on
more recent clangs ("moving a local object in a return statement
prevents copy elision")
We just need to make the `Expected` ctor taking an `Error` take it as a
r-value reference.
[1] https://lab.llvm.org/buildbot/#/builders/54/builds/10505
217668f641