Fix spans for attributes

They used to be one token too long, so you'd see things like
```
rust/rust/test.rs:1:1: 2:2 warning: unused attribute,
rust/rust/test.rs:1 #![foo]
rust/rust/test.rs:2 #![bar]
```
instead of
```
test.rs:1:1: 1:8 warning: unused attribute, #[warn(unused_attribute)] on
by default
test.rs:1 #![foo]
          ^~~~~~~
```
auto merge of #15621 : sfackler/rust/attr-span, r=cmr

They used to be one token too long, so you'd see things like
```
rust/rust/test.rs:1:1: 2:2 warning: unused attribute,
rust/rust/test.rs:1 #![foo]
rust/rust/test.rs:2 #![bar]
```
instead of
```
test.rs:1:1: 1:8 warning: unused attribute, #[warn(unused_attribute)] on
by default
test.rs:1 #![foo]
          ^~~~~~~
```