Fix macro expansion backtrace diagnostics

[jseyfried]

Fixes#33704.
r? @nikomatsakis

[jseyfried]

For example, consider:

macro_rules! m {() => {{ error }}}fnmain(){m!();}

Currently, the diagnostics are:

bt.rs:2 () => { { error } }
^~~~~
bt.rs:2:15: 2:20 note: in this expansion of m! (defined in bt.rs)
bt.rs:2:15: 2:20 help: run `rustc --explain E0425` to see a detailed explanation

After this PR, the diagnostics will be the same as on beta:

bt.rs:2:15: 2:20 error: unresolved name `error` [E0425]
bt.rs:2 () => { { error } }
^~~~~
bt.rs:4:13: 4:18 note: in this expansion of m! (defined in bt.rs)
bt.rs:2:15: 2:20 help: run `rustc --explain E0425` to see a detailed explanation

[nikomatsakis]

@jseyfried can we add a test for this?

[jseyfried]

@nikomatsakis I don't know -- I'm not very familiar with the recent changes to the testing framework.

[nikomatsakis]

@bors r+ rollup

[bors]

📌 Commit f8e9c15 has been approved by nikomatsakis

[nikomatsakis]

we could make a ui test, but maybe not worth it -- @jonathandturner and I did plan to revisit how macro backtraces are presented, maybe we'll do it then