The fix for 23433271 also fixed the crash on 22058555, add its testcase so

we don't regress on it in the future.  Yes, the diagnostic is lousy, I'll
repurpose the second radar to improve it.

Author: lattner

test/Constraints/diagnostics.swift

@@ -641,5 +641,11 @@ func segfault23433271(a : UnsafeMutablePointer<Void>) {
   f23433271(a[0])  // expected-error {{cannot subscript a value of type 'UnsafeMutablePointer<Void>' (aka 'UnsafeMutablePointer<()>')}}
 }
 
-
+// <rdar://problem/22058555> crash in cs diags in withCString
+func r22058555() {
+  var firstChar: UInt8 = 0
+  "abc".withCString { chars in
+    firstChar = chars[0]  // expected-error {{cannot subscript a value of type 'UnsafePointer<Int8>'}}
+  }
+}