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.

1 files changed, 2 insertions, 3 deletions

diff --git a/src/test/ui/span/issue-23729.stderr b/src/test/ui/span/issue-23729.stderr
index 701576ff6f4..d9f4bacce35 100644
--- a/src/test/ui/span/issue-23729.stderr
+++ b/src/test/ui/span/issue-23729.stderr
@@ -1,15 +1,14 @@
 error[E0046]: not all trait items implemented, missing: `Item`
   --> $DIR/issue-23729.rs:20:9
    |
-20 |           impl Iterator for Recurrence {
-   |  _________^ starting here...
+20 | /         impl Iterator for Recurrence {
 21 | |             //~^ ERROR E0046
 22 | |             //~| NOTE missing `Item` in implementation
 23 | |             //~| NOTE `Item` from trait: `type Item;`
 ...  |
 36 | |             }
 37 | |         }
-   | |_________^ ...ending here: missing `Item` in implementation
+   | |_________^ missing `Item` in implementation
    |
    = note: `Item` from trait: `type Item;`