Update the existing UI tests to reflect diagnostic changes in NLL.

Author: pnkfelix

src/test/ui/asm-out-assign-imm.nll.stderr

@@ -0,0 +1,14 @@
+error[E0384]: cannot assign twice to immutable variable `x`
+  --> $DIR/asm-out-assign-imm.rs:33:9
+   |
+LL |     let x: isize;
+   |         - consider changing this to `mut x`
+LL |     x = 1;
+   |     ----- first assignment to `x`
+...
+LL |         asm!("mov $1, $0" : "=r"(x) : "r"(5));
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot assign twice to immutable variable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0384`.

src/test/ui/borrowck/immutable-arg.stderr

@@ -10,7 +10,7 @@ error[E0384]: cannot assign to immutable argument `_x` (Mir)
   --> $DIR/immutable-arg.rs:14:5
    |
 LL | fn foo(_x: u32) {
-   |        -- argument not declared as `mut`
+   |        -- consider changing this to `mut _x`
 LL |     _x = 4;
    |     ^^^^^^ cannot assign to immutable argument
 

src/test/ui/command-line-diagnostics.nll.stderr

@@ -2,7 +2,9 @@ error[E0384]: cannot assign twice to immutable variable `x`
   --> $DIR/command-line-diagnostics.rs:16:5
    |
 LL |     let x = 42;
-   |             -- first assignment to `x`
+   |         -   -- first assignment to `x`
+   |         |
+   |         consider changing this to `mut x`
 LL |     x = 43;
    |     ^^^^^^ cannot assign twice to immutable variable
 

src/test/ui/did_you_mean/issue-35937.nll.stderr

@@ -10,15 +10,20 @@ error[E0384]: cannot assign twice to immutable variable `s.x`
   --> $DIR/issue-35937.rs:26:5
    |
 LL |     let s = S { x: 42 };
-   |             ----------- first assignment to `s.x`
+   |         -   ----------- first assignment to `s.x`
+   |         |
+   |         consider changing this to `mut s`
 LL |     s.x += 1; //~ ERROR cannot assign
    |     ^^^^^^^^ cannot assign twice to immutable variable
 
 error[E0384]: cannot assign twice to immutable variable `s.x`
   --> $DIR/issue-35937.rs:30:5
    |
 LL | fn bar(s: S) {
-   |        - first assignment to `s.x`
+   |        -
+   |        |
+   |        first assignment to `s.x`
+   |        consider changing this to `mut s`
 LL |     s.x += 1; //~ ERROR cannot assign
    |     ^^^^^^^^ cannot assign twice to immutable variable
 

src/test/ui/lifetime-errors/ex3-both-anon-regions-one-is-struct-2.nll.stderr

@@ -18,7 +18,7 @@ error[E0384]: cannot assign to immutable argument `y`
   --> $DIR/ex3-both-anon-regions-one-is-struct-2.rs:14:5
    |
 LL | fn foo(mut x: Ref, y: &u32) {
-   |                    - argument not declared as `mut`
+   |                    - consider changing this to `mut y`
 LL |     y = x.b; //~ ERROR lifetime mismatch
    |     ^^^^^^^ cannot assign to immutable argument
 

src/test/ui/lifetime-errors/liveness-assign-imm-local-notes.stderr

@@ -32,6 +32,9 @@ LL |             x = 2;      //~ ERROR (Ast) [E0384]
 error[E0384]: cannot assign twice to immutable variable `x` (Mir)
   --> $DIR/liveness-assign-imm-local-notes.rs:23:9
    |
+LL |     let x;
+   |         - consider changing this to `mut x`
+...
 LL |         x = 2;
    |         ----- first assignment to `x`
 LL |         x = 3;      //~ ERROR (Ast) [E0384]
@@ -40,6 +43,9 @@ LL |         x = 3;      //~ ERROR (Ast) [E0384]
 error[E0384]: cannot assign twice to immutable variable `x` (Mir)
   --> $DIR/liveness-assign-imm-local-notes.rs:35:13
    |
+LL |         let x;
+   |             - consider changing this to `mut x`
+...
 LL |             x = 2;
    |             ----- first assignment to `x`
 LL |             x = 3;      //~ ERROR (Ast) [E0384]
@@ -48,12 +54,18 @@ LL |             x = 3;      //~ ERROR (Ast) [E0384]
 error[E0384]: cannot assign twice to immutable variable `x` (Mir)
   --> $DIR/liveness-assign-imm-local-notes.rs:45:13
    |
+LL |     let x;
+   |         - consider changing this to `mut x`
+...
 LL |             x = 1;      //~ ERROR (Ast) [E0384]
    |             ^^^^^ cannot assign twice to immutable variable
 
 error[E0384]: cannot assign twice to immutable variable `x` (Mir)
   --> $DIR/liveness-assign-imm-local-notes.rs:48:13
    |
+LL |     let x;
+   |         - consider changing this to `mut x`
+...
 LL |             x = 1;      //~ ERROR (Ast) [E0384]
    |             ----- first assignment to `x`
 ...