[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/string_view b/include/string_view
index bc92dd5..3177fcd 100644
--- a/include/string_view
+++ b/include/string_view
@@ -182,6 +182,7 @@
 #include <__string>
 #include <iosfwd>
 #include <algorithm>
+#include <compare>
 #include <iterator>
 #include <limits>
 #include <stdexcept>