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.

llvm-svn: 329460
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 0913ca197855626c93fdae4184ae80ff5a26503a
diff --git a/include/module.modulemap b/include/module.modulemap
index cf92fa0..4739386 100644
--- a/include/module.modulemap
+++ b/include/module.modulemap
@@ -243,6 +243,10 @@
     header "codecvt"
     export *
   }
+  module compare {
+    header "compare"
+    export *
+  }
   module complex {
     header "complex"
     export *