SymbolGraph: Don't unconditionally add edge targets to the graph

Edge targets might point outside the module, so don't include them
unconditionally.

rdar://58876107

Author: bitjammer

lib/SymbolGraphGen/SymbolGraph.cpp

@@ -139,7 +139,12 @@ void SymbolGraph::recordEdge(const ValueDecl *Source,
   }
 
   Nodes.insert(Source);
-  Nodes.insert(Target);
+  if (Target->getModuleContext() != &M) {
+    // Don't claim a symbol just because we have a relationship to it.
+    // For example, if we conform to `Sequence`, that symbol's node should be
+    // under Swift, not this module.
+    Nodes.insert(Target);
+  }
 
   Edges.insert({this, Kind, Source, Target});
 }

test/SymbolGraph/Module/Extension.swift

@@ -18,3 +18,9 @@ public extension String {
 // CHECK: "kind": "memberOf"
 // CHECK-NEXT: "source": "s:SS9ExtensionE9somethingSSvp"
 // CHECK-NEXT: "target": "s:SS"
+
+// Extending `String` creates a memberOf relationship above.
+// However, it should not be included as a node because `String`
+// is owned by the Swift module.
+// rdar://58876107
+// CHECK-NOT: "precise": "s:SS"