Elide all artificial copy constructors, because
they are probably trivial.  This means that we
don't confuse Clang about whether a class is
trivially copy constructible.  It can figure
that out itself as long as we don't explicitly
feed it the constructors.

If the class is trivially copy-constructible,
this can change the ABI that Clang uses to call
functions that return that class (e.g., by making
the object be returned in a register), so this
is quite important for correctness.

<rdar://problem/13457741>