[FixCode] Don't accept fixit that inserts commas; they break placeholders (#2869)

These fixits interact poorly with code that has <#editor placeholder#>s
in it, inserting commas inside the placeholder.

rdar://problem/26623964

Author: benlangmuir

lib/FrontendTool/FrontendTool.cpp

@@ -555,7 +555,8 @@ class JSONFixitWriter : public DiagnosticConsumer {
       return true;
 
     // Do not add a semi as it is wrong in most cases during migration
-    if (Info.ID == diag::statement_same_line_without_semi.ID)
+    if (Info.ID == diag::statement_same_line_without_semi.ID ||
+        Info.ID == diag::expected_separator.ID)
       return false;
     // The following interact badly with the swift migrator, they are undoing
     // migration of arguments to preserve the no-label for first argument.

test/FixCode/fixits-apply.swift

@@ -142,3 +142,9 @@ var graph: Graph3
 
 class GraphNoFix<NodeType : SomeProt> {}
 var graph: GraphNoFix
+
+func evilCommas(s: String) {
+  _ = s[s.startIndex..<<#editorplaceholder#>]
+  _ = true ? s[s.startIndex..<<#editorplaceholder#>] : ""
+  _ = [s.startIndex..<<#editorplaceholder#>]
+}

test/FixCode/fixits-apply.swift.result

@@ -145,3 +145,9 @@ var graph: Graph3<AnyObject>
 
 class GraphNoFix<NodeType : SomeProt> {}
 var graph: GraphNoFix
+
+func evilCommas(s: String) {
+  _ = s[s.startIndex..<<#editorplaceholder#>]
+  _ = true ? s[s.startIndex..<<#editorplaceholder#>] : ""
+  _ = [s.startIndex..<<#editorplaceholder#>]
+}