[code-completion] Add a new custom completion context for a for-each sequence

For normal completions it behaves the same as PostfixExprBeginning, but
it provides a hook for clients to provide a custom completion for this
position.  For example, you might want to a x ..< y snippet in this
position.

rdar://problem/29910383

tools/SourceKit/lib/SwiftLang/CodeCompletionOrganizer.cpp

@@ -184,6 +184,12 @@ bool SourceKit::CodeCompletion::addCustomCompletions(
         addCompletion(custom);
       }
       break;
+    case CompletionKind::ForEachSequence:
+      if (custom.Contexts.contains(CustomCompletionInfo::ForEachSequence)) {
+        changed = true;
+        addCompletion(custom);
+      }
+      break;
     case CompletionKind::TypeSimpleBeginning:
       if (custom.Contexts.contains(CustomCompletionInfo::Type)) {
         changed = true;