[NFC][libc++] Moves ios_base's forward declaration. (#88027)

According to our synopsis it belonged to ios_fwd. This is not true in
the C++11 version of the Standard, I did not validate against C++98.

Moving this to ios's forward where it's declared in the standard allows
removing a module quirk. An earlier removal of std::vectors forward
declaration allows to remove all quirks for the iosfwd module part.

Since iosfwd includes __fwd/ios.h this does not change the required
includes.