Implement P0768r1: Library support for the Spaceship Operator.

this patch adds the <compare> header and implements all of it
except for [comp.alg].

As I understand it, the header is needed by the compiler in
when implementing the semantics of operator<=>. For that reason
I feel it's important to land this header early, despite
all compilers lacking support.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@329460 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/double_include.sh.cpp

@@ -41,6 +41,7 @@
 #include <clocale>
 #include <cmath>
 #include <codecvt>
+#include <compare>
 #include <complex>
 #include <complex.h>
 #include <condition_variable>