[cxx-interop] Don't synthesize conformance to invalid protocols.

Author: zoecarver

lib/ClangImporter/ImportDecl.cpp

@@ -429,8 +429,13 @@ void ClangImporter::Implementation::addSynthesizedProtocolAttrs(
   auto &ctx = nominal->getASTContext();
 
   for (auto kind : synthesizedProtocolAttrs) {
-    nominal->getAttrs().add(
-        new (ctx) SynthesizedProtocolAttr(ctx.getProtocol(kind), this, isUnchecked));
+    // This is unfortunately not an error because some test use mock protocols.
+    // If those tests were updated, we could assert that
+    // ctx.getProtocol(kind) != nulltpr which would be nice.
+    if (auto proto = ctx.getProtocol(kind))
+      nominal->getAttrs().add(
+          new (ctx) SynthesizedProtocolAttr(ctx.getProtocol(kind), this,
+                                            isUnchecked));
   }
 }