Fixes#75002. Found while running libc++'s tests with MSVC's STL.
This is a superset of #74961 that also fixes the product code
and adds a regression test. Thanks again, @cpplearner!
To summarize: `views::split` and `views::lazy_split` aren't unary,
aren't range adaptor **closure** objects, and can't be piped. However,
\[range.adaptor.object\]/8 says that `views::split(pattern)` and
`views::lazy_split(pattern)` produce unary, pipeable, range adaptor
closure objects.
This PR adjusts the test coverage accordingly, allowing it to portably
pass for libc++ and MSVC's STL.
869133771a