Remove a bunch of LIBCPP_CXX03_LANG. This is the result of a
rabbithole to re-eliminate the workaround I introduced into
std::cref in D117953. It turns out that Clang's C++03 mode
(the only compiler we care about C++03 for) now supports all
the things we were originally eschewing via LIBCPP_CXX03_LANG;
we can fully support these reference_wrapper features in
C++03 mode, and un-XFAIL the relevant tests.
Drive-by constexprify a few more tests.
Differential Revision: https://reviews.llvm.org/D117974
c99a585399