[c++2a] P0515R3: Support for overloaded operator<=>.

No CodeGen support for MSABI yet, we don't know how to mangle this there.

llvm-svn: 319513
diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
index fa62798..8680d3d 100644
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -8692,6 +8692,9 @@
     OpBuilder.addGenericBinaryArithmeticOverloads();
     break;
 
+  case OO_Spaceship:
+    llvm_unreachable("<=> expressions not supported yet");
+
   case OO_Percent:
   case OO_Caret:
   case OO_Pipe: