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.