[libc++] Mark string operator+ _LIBCPP_FUNC_VIS

It has an extern template instantiation declaration in the headers and a
corresponding instantiation definition in the library, so we must mark
it with _LIBCPP_FUNC_VIS to make it available outside the library.

This doesn't cause any ABI changes as-is since we don't build libc++
with hidden visibility (so the function is exported anyway). It's needed
for building libc++ with hidden visibility, however.

Clarify the Windows behavior for extern function templates while I'm
here, since this exercises that behavior.

llvm-svn: 307966