[libc++] Header inclusion tests.

As mandated by the Standard's various synopses, e.g. [iterator.synopsis].
Searching the TeX source for '#include' is a good way to find all of these
mandates.

The new tests are all autogenerated by utils/generate_header_inclusion_tests.py.
I was SHOCKED by how many mandates there are, and how many of them
libc++ wasn't conforming with.

Differential Revision: https://reviews.llvm.org/D99309

GitOrigin-RevId: 2d0f1fa472a99dbb46735623363fe918feeda795
diff --git a/include/iterator b/include/iterator
index c02f523..54ea2aa 100644
--- a/include/iterator
+++ b/include/iterator
@@ -418,6 +418,8 @@
 #include <iosfwd> // for forward declarations of vector and string.
 #include <__functional_base>
 #include <type_traits>
+#include <compare>
+#include <concepts>
 #include <cstddef>
 #include <initializer_list>
 #include <__memory/base.h>