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.
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