[FixCode] Pick up fixit for let->var change (#2848)

This is an important fixit, but was lost because it is on the note
instead of the error.

rdar://problem/26593117

Author: benlangmuir

lib/FrontendTool/FrontendTool.cpp

@@ -575,6 +575,7 @@ class JSONFixitWriter : public DiagnosticConsumer {
         Info.ID == diag::forced_downcast_noop.ID ||
         Info.ID == diag::variable_never_mutated.ID ||
         Info.ID == diag::function_type_no_parens.ID ||
+        Info.ID == diag::convert_let_to_var.ID ||
         Info.ID == diag::parameter_extraneous_double_up.ID)
       return true;
     return false;

test/FixCode/fixits-apply.swift

@@ -112,3 +112,13 @@ func ftest3(_ fd: CInt) {
 func ftest4(_ fd: UInt) {
   recit(fd)
 }
+
+func letToVar1() {
+  let x = 1
+  if x == 2 {
+    x += 3
+  }
+  let y = ""
+  y.append("as")
+  y.append("df")
+}

test/FixCode/fixits-apply.swift.result

@@ -115,3 +115,13 @@ func ftest3(_ fd: CInt) {
 func ftest4(_ fd: UInt) {
   recit(Int32(fd))
 }
+
+func letToVar1() {
+  var x = 1
+  if x == 2 {
+    x += 3
+  }
+  var y = ""
+  y.append("as")
+  y.append("df")
+}