[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

libcxx/test/std/strings/basic.string/string.cons/string_view_deduction.pass.cpp

@@ -35,7 +35,7 @@
 
 #include "test_macros.h"
 #include "test_allocator.h"
-#include "../input_iterator.h"
+#include "../cpp17_input_iterator.h"
 #include "min_allocator.h"
 
 int main(int, char**)