[libcxx][nfc] prefixes test type `input_iterator` with `cpp17_`

C++20 revised the definition of what it means to be an iterator. While
all _Cpp17InputIterators_ satisfy `std::input_iterator`, the reverse
isn't true. D100271 introduces a new test adaptor to accommodate this
new definition (`cpp20_input_iterator`).

In order to help readers immediately distinguish which input iterator
adaptor is _Cpp17InputIterator_, the current `input_iterator` adaptor
has been prefixed with `cpp17_`.

Differential Revision: https://reviews.llvm.org/D101242