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

include/module.modulemap

@@ -243,6 +243,10 @@ module std [system] {
     header "codecvt"
     export *
   }
+  module compare {
+    header "compare"
+    export *
+  }
   module complex {
     header "complex"
     export *