[libc++] Hoist explicit instantiation above implicit. NFC

The string literal operators have implicit instantiations of
basic_string<char> and basic_string<wchar>, which prevent the dllimport
on the subsequent explicit instantiation declarations from having an
effect. Hoisting the explicit instantiations above the implicit ones
fixes the issue.

I think it's pretty unfortunate that the ordering has such an effect,
and I'd fixed the same issue for dllexport in r288682. dllimport is more
complicated from a codegen perspective, however, and clang's behavior of
ignoring the dllimport when there's a previous implicit instantiation is
consistent with cl, so changing the order is our only recourse.

llvm-svn: 306632