[libc++] Enable `explicit` conversion operators, even in C++03 mode.

C++03 didn't support `explicit` conversion operators;
but Clang's C++03 mode does, as an extension, so we can use it.
This lets us make the conversion explicit in `std::function` (even in '03),
and remove some silly metaprogramming in `std::basic_ios`.

Drive-by improvements to the tests for these operators, in addition
to making sure all these tests also run in `c++03` mode.

Differential Revision: https://reviews.llvm.org/D104682