libstdc++: Add missing friend declarations in some range adaptors

Some of the range adaptors have distinct constant and non-constant
iterator/sentinel types, along with converting constructors that can convert a
non-constant iterator/sentinel to a constant iterator/sentinel.  This patch adds
the missing appropriate friend declarations in order to make these converting
constructors well formed.

Strictly speaking it seems the friendship relations don't need to go both ways
-- we could get away with declaring e.g. friend _Iterator<false>; instead of
friend _Iterator<!_Const>; but both reference implementations seem to use the
latter symmetric form anyway.

libstdc++-v3/ChangeLog:

	* include/std/ranges (transform_view::_Iterator<_Const>): Befriend
	_Iterator<!_Const>.
	(transform_view::_Sentinel<_Const>): Befriend _Sentinel<!_Const>.
	(take_view::_Sentinel<_Const>): Likewise.
	(take_while_view::_Sentinel<_Const>): Likewise.
	(split_view::_OuterIter<_Const>): Befriend _OuterIter<!_Const>.
	* testsuite/std/ranges/adaptors/split.cc: Augment test.
	* testsuite/std/ranges/adaptors/take.cc: Augment test.
	* testsuite/std/ranges/adaptors/take_while.cc: Augment test.
	* testsuite/std/ranges/adaptors/transform.cc: Augment test.

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