[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.

Author: mordante

libcxx/include/__fwd/ios.h

@@ -18,6 +18,8 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+class _LIBCPP_EXPORTED_FROM_ABI ios_base;
+
 template <class _CharT, class _Traits = char_traits<_CharT> >
 class _LIBCPP_TEMPLATE_VIS basic_ios;
 

libcxx/include/iosfwd

@@ -25,7 +25,6 @@ template<>            struct char_traits<wchar_t>;
 
 template<class T>     class allocator;
 
-class ios_base;
 template <class charT, class traits = char_traits<charT> > class basic_ios;
 
 template <class charT, class traits = char_traits<charT> > class basic_streambuf;
@@ -124,8 +123,6 @@ using wosyncstream = basic_osyncstream<wchar_t>;  // C++20
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-class _LIBCPP_EXPORTED_FROM_ABI ios_base;
-
 template <class _CharT, class _Traits = char_traits<_CharT> >
 class _LIBCPP_TEMPLATE_VIS istreambuf_iterator;
 template <class _CharT, class _Traits = char_traits<_CharT> >

libcxx/utils/libcxx/test/modules.py

@@ -26,8 +26,6 @@
 # The operators are added for private types like __iom_t10.
 SkipDeclarations["iomanip"] = ["std::operator<<", "std::operator>>"]
 
-SkipDeclarations["iosfwd"] = ["std::ios_base", "std::vector"]
-
 # This header also provides declarations in the namespace that might be
 # an error.
 SkipDeclarations["filesystem"] = [