This follows on from D104172 (which is the same idea, STL container edition). The idea is to remove #ifs that have become unnecessary due to Clang's C++03 mode gaining more C++11 features as extensions. We need to keep libc++ building with clang++ -std=c++03 and with g++ -std=c++11, but we don't claim to build in pedantic C++03 mode — anything Clang supports with a warning, we can use!
[libc++] Remove some _LIBCPP_CXX03_LANG from iostreams headers. With the STL containers, I didn't enable move operations in C++03 mode because that would change the overload resolution for things that today are copy operations. With iostreams, though, the copy operations aren't present at all, and so I see no problem with enabling move operations even in (Clang's greatly extended) C++03 mode. Clang's C++03 mode does not support delegating constructors.