Reduce visual clutter of multiline start when possible

When a span starts on a line with nothing but whitespace to the left,
and there are no other annotations in that line, simplify the visual
representation of the span.

Go from:

```rust
error[E0072]: recursive type `A` has infinite size
 --> file2.rs:1:1
  |
1 |   struct A {
  |  _^ starting here...
2 | |     a: A,
3 | | }
  | |_^ ...ending here: recursive type has infinite size
  |
```

To:

```rust
error[E0072]: recursive type `A` has infinite size
 --> file2.rs:1:1
  |
1 | / struct A {
2 | |     a: A,
3 | | }
  | |_^ recursive type has infinite size
```

Remove `starting here...`/`...ending here` labels from all multiline
diagnostics.

src/libsyntax/test_snippet.rs

@@ -128,9 +128,9 @@ error: foo
  --> test.rs:2:10
   |
 2 |   fn foo() {
-  |  __________^ starting here...
+  |  __________^
 3 | | }
-  | |_^ ...ending here: test
+  | |_^ test
 
 "#);
 }
@@ -161,11 +161,11 @@ error: foo
  --> test.rs:2:10
   |
 2 |   fn foo() {
-  |  __________^ starting here...
+  |  __________^
 3 | |
 4 | |
 5 | |   }
-  | |___^ ...ending here: test
+  | |___^ test
 
 "#);
 }
@@ -207,14 +207,14 @@ error: foo
  --> test.rs:3:3
   |
 3 |      X0 Y0
-  |  ____^__- starting here...
+  |  ____^__-
   | | ___|
-  | ||   starting here...
+  | ||
 4 | ||   X1 Y1
 5 | ||   X2 Y2
-  | ||____^__- ...ending here: `Y` is a good letter too
+  | ||____^__- `Y` is a good letter too
   |  |____|
-  |       ...ending here: `X` is a good letter
+  |       `X` is a good letter
 
 "#);
 }
@@ -256,13 +256,13 @@ error: foo
  --> test.rs:3:3
   |
 3 |      X0 Y0
-  |  ____^__- starting here...
+  |  ____^__-
   | | ___|
-  | ||   starting here...
+  | ||
 4 | ||   Y1 X1
-  | ||____-__^ ...ending here: `X` is a good letter
+  | ||____-__^ `X` is a good letter
   | |_____|
-  |       ...ending here: `Y` is a good letter too
+  |       `Y` is a good letter too
 
 "#);
 }
@@ -306,13 +306,13 @@ error: foo
  --> test.rs:3:6
   |
 3 |      X0 Y0 Z0
-  |   ______^ starting here...
+  |   ______^
 4 |  |   X1 Y1 Z1
-  |  |_________- starting here...
+  |  |_________-
 5 | ||   X2 Y2 Z2
-  | ||____^ ...ending here: `X` is a good letter
+  | ||____^ `X` is a good letter
 6 | |    X3 Y3 Z3
-  | |_____- ...ending here: `Y` is a good letter too
+  | |_____- `Y` is a good letter too
 
 "#);
 }
@@ -366,16 +366,16 @@ error: foo
  --> test.rs:3:3
   |
 3 |       X0 Y0 Z0
-  |  _____^__-__- starting here...
+  |  _____^__-__-
   | | ____|__|
-  | || ___|  starting here...
-  | |||   starting here...
+  | || ___|
+  | |||
 4 | |||   X1 Y1 Z1
 5 | |||   X2 Y2 Z2
-  | |||____^__-__- ...ending here: `Z` label
+  | |||____^__-__- `Z` label
   |  ||____|__|
-  |   |____|  ...ending here: `Y` is a good letter too
-  |        ...ending here: `X` is a good letter
+  |   |____|  `Y` is a good letter too
+  |        `X` is a good letter
 
 "#);
 }
@@ -430,17 +430,17 @@ error: foo
  --> test.rs:3:6
   |
 3 |      X0 Y0 Z0
-  |   ______^ starting here...
+  |   ______^
 4 |  |   X1 Y1 Z1
-  |  |____^_- starting here...
+  |  |____^_-
   | ||____|
-  | |     ...ending here: `X` is a good letter
+  | |     `X` is a good letter
 5 | |    X2 Y2 Z2
-  | |____-______- ...ending here: `Y` is a good letter too
+  | |____-______- `Y` is a good letter too
   |  ____|
-  | |    starting here...
+  | |
 6 | |    X3 Y3 Z3
-  | |________- ...ending here: `Z`
+  | |________- `Z`
 
 "#);
 }
@@ -458,7 +458,7 @@ fn foo() {
     vec![
         SpanLabel {
             start: Position {
-                string: "Y0",
+                string: "X0",
                 count: 1,
             },
             end: Position {
@@ -481,16 +481,15 @@ fn foo() {
     ],
     r#"
 error: foo
- --> test.rs:3:6
+ --> test.rs:3:3
   |
-3 |     X0 Y0 Z0
-  |  ______^ starting here...
+3 | /   X0 Y0 Z0
 4 | |   X1 Y1 Z1
-  | |____^ ...ending here: `X` is a good letter
+  | |____^ `X` is a good letter
 5 |     X2 Y2 Z2
-  |  ______- starting here...
+  |  ______-
 6 | |   X3 Y3 Z3
-  | |__________- ...ending here: `Y` is a good letter too
+  | |__________- `Y` is a good letter too
 
 "#);
 }
@@ -534,14 +533,14 @@ error: foo
  --> test.rs:3:6
   |
 3 |      X0 Y0 Z0
-  |   ______^ starting here...
+  |   ______^
 4 |  |   X1 Y1 Z1
-  |  |____^____- starting here...
+  |  |____^____-
   | ||____|
-  | |     ...ending here: `X` is a good letter
+  | |     `X` is a good letter
 5 | |    X2 Y2 Z2
 6 | |    X3 Y3 Z3
-  | |___________- ...ending here: `Y` is a good letter too
+  | |___________- `Y` is a good letter too
 
 "#);
 }
@@ -982,18 +981,18 @@ error: foo
   --> test.rs:3:6
    |
 3  |      X0 Y0 Z0
-   |   ______^ starting here...
+   |   ______^
 4  |  |   X1 Y1 Z1
-   |  |____^____- starting here...
+   |  |____^____-
    | ||____|
-   | |     ...ending here: `X` is a good letter
+   | |     `X` is a good letter
 5  | |  1
 6  | |  2
 7  | |  3
 ...  |
 15 | |    X2 Y2 Z2
 16 | |    X3 Y3 Z3
-   | |___________- ...ending here: `Y` is a good letter too
+   | |___________- `Y` is a good letter too
 
 "#);
 }
@@ -1047,21 +1046,21 @@ error: foo
   --> test.rs:3:6
    |
 3  |      X0 Y0 Z0
-   |   ______^ starting here...
+   |   ______^
 4  |  | 1
 5  |  | 2
 6  |  | 3
 7  |  |   X1 Y1 Z1
-   |  |_________- starting here...
+   |  |_________-
 8  | || 4
 9  | || 5
 10 | || 6
 11 | ||   X2 Y2 Z2
-   | ||__________- ...ending here: `Z` is a good letter too
+   | ||__________- `Z` is a good letter too
 ...   |
 15 |  | 10
 16 |  |   X3 Y3 Z3
-   |  |_______^ ...ending here: `Y` is a good letter
+   |  |_______^ `Y` is a good letter
 
 "#);
 }