iomanip.h: Use __extension__ for `extern' explicit template instantiations.

iomanip.h: Use __extension__ for `extern' explicit template instantiations.

	* iomanip.h: Use __extension__ for `extern' explicit template
	instantiations.
	* sinst.cc: Don't explicitly instantiation string_char_traits<char>.
	* cinst.cc: Likewiwse, for complex<float>, complex<double>,
	complex<long double>.
	* extend.texi: Remove description of extension to explicit
	instantiation that is now endorsed by standard C++.
	* decl2.c (grok_array_decl): Add comment.
	(mark_used): Don't instantiate an explicit instantiation.
	* friend.c (make_friend_class): Remove bogus comment.  Fix check
	for partial specializations.
	* pt.c (check_explicit_specialization): Don't
	SET_DECL_EXPLICIT_INSTANTIATION here.
	(mark_decl_instantiated): Or here.
	(do_decl_instantiation): Do it here, instead.  Add checks for
	duplicate explicit instantiations, etc.  Tidy.
	(do_type_instantiation): Likewise.
	(instantiate_decl): Improve comments.  Complain about explicit
	instantiations where no definition is available.
	* cp-tree.h (ansi_null_node): Remove.
	* call.c (build_over_call): Warn about converting NULL to an
	arithmetic type.
	* cvt.c (build_expr_type_conversion): Likewise.  Use
	null_ptr_cst_p instead of expanding it inline.
	* decl.c (ansi_null_node): Remove.
	(init_decl_processing): Make null_node always have integral type.
	* except.c (build_throw): Warn about converting NULL to an
	arithmetic type.
	* lex.c (init_parse): Remove handling of ansi_null_node.
	* pt.c (type_unification_real): Don't convert NULL to void* type.
	* typeck.c (build_binary_op_nodefault): Fix NULL warnings.
	(convert_for_assignment): Warn about converting NULL to an
	arithmetic type.
	(convert_for_initialization): Likewise.

From-SVN: r21915