Add -Z borrowck=migrate flag, use it to link NLL up to AST-borrowck.

src/librustc_errors/diagnostic.rs

@@ -99,6 +99,25 @@ impl Diagnostic {
         }
     }
 
+    pub fn is_error(&self) -> bool {
+        match self.level {
+            Level::Bug |
+            Level::Fatal |
+            Level::PhaseFatal |
+            Level::Error |
+            Level::FailureNote => {
+                true
+            }
+
+            Level::Warning |
+            Level::Note |
+            Level::Help |
+            Level::Cancelled => {
+                false
+            }
+        }
+    }
+
     /// Cancel the diagnostic (a structured diagnostic must either be emitted or
     /// canceled or it will panic when dropped).
     pub fn cancel(&mut self) {

src/librustc_errors/diagnostic_builder.rs

@@ -100,25 +100,6 @@ impl<'a> DiagnosticBuilder<'a> {
         buffered_diagnostics.push(diagnostic);
     }
 
-    pub fn is_error(&self) -> bool {
-        match self.level {
-            Level::Bug |
-            Level::Fatal |
-            Level::PhaseFatal |
-            Level::Error |
-            Level::FailureNote => {
-                true
-            }
-
-            Level::Warning |
-            Level::Note |
-            Level::Help |
-            Level::Cancelled => {
-                false
-            }
-        }
-    }
-
     /// Convenience function for internal use, clients should use one of the
     /// span_* methods instead.
     pub fn sub<S: Into<MultiSpan>>(