Update tests for new NLL mutability errors

src/test/compile-fail/E0594.rs

@@ -15,5 +15,5 @@ static NUM: i32 = 18;
 
 fn main() {
     NUM = 20; //[ast]~ ERROR E0594
-              //[mir]~^ ERROR cannot assign to immutable item `NUM`
+              //[mir]~^ ERROR cannot assign to immutable static item `NUM`
 }

src/test/compile-fail/borrowck/borrowck-assign-to-constants.rs

@@ -16,5 +16,5 @@ static foo: isize = 5;
 fn main() {
     // assigning to various global constants
     foo = 6; //[ast]~ ERROR cannot assign to immutable static item
-             //[mir]~^ ERROR cannot assign to immutable item `foo`
+             //[mir]~^ ERROR cannot assign to immutable static item `foo`
 }

src/test/compile-fail/cannot-mutate-captured-non-mut-var.rs

@@ -22,10 +22,10 @@ fn main() {
     let x = 1;
     to_fn_once(move|| { x = 2; });
     //[ast]~^ ERROR: cannot assign to immutable captured outer variable
-    //[mir]~^^ ERROR: cannot assign to immutable item `x`
+    //[mir]~^^ ERROR: cannot assign to `x`, as it is not declared as mutable
 
     let s = std::io::stdin();
     to_fn_once(move|| { s.read_to_end(&mut Vec::new()); });
     //[ast]~^ ERROR: cannot borrow immutable captured outer variable
-    //[mir]~^^ ERROR: cannot borrow immutable item `s` as mutable
+    //[mir]~^^ ERROR: cannot borrow `s` as mutable, as it is not declared as mutable
 }

src/test/compile-fail/nll/constant-thread-locals-issue-47053.rs

@@ -17,5 +17,5 @@
 static FOO: isize = 5;
 
 fn main() {
-    FOO = 6; //~ ERROR cannot assign to immutable item `FOO` [E0594]
+    FOO = 6; //~ ERROR cannot assign to immutable static item `FOO` [E0594]
 }