Auto merge of #45545 - durka:macro-backtrace, r=nrc

show macro backtrace with -Z flag

Fixes #39413 by adding a facility to restore the "old school" macro expansion backtraces (previously removed in 61865384b8).

The restored functionality is accessed through the flag `-Z external-macro-backtrace`. Errors showing the truncated backtraces will suggest this flag.

### Example

Code: <details>
`a/src/lib.rs`
```rust
#[macro_export]
macro_rules! a {
    () => { a!(@) };
    (@) => { a!(@@) };
    (@@) => {
        syntax error;
    }
}
```
`b/src/main.rs`
```rust
#[macro_use] extern crate a;

macro_rules! b {
    () => { b!(@) };
    (@) => { b!(@@) };
    (@@) => {
        syntax error;
    }
}

fn main() {
    a!();
    b!();
}
```
</details>

<br/><br/>
Running without env var (note: first error is from remote macro, second from local macro):

<details>

```
$ cargo +custom run
   Compiling b v0.1.0
error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `error`
  --> src/main.rs:12:5
   |
12 |     a!();
   |     ^^^^^
   |     |
   |     expected one of 8 possible tokens here
   |     unexpected token
   |
   = note: this error originates in a macro outside of the current crate (run with RUST_MACRO_BACKTRACE=1 for more info)

error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `error`
  --> src/main.rs:7:16
   |
7  |         syntax error;
   |               -^^^^^ unexpected token
   |               |
   |               expected one of 8 possible tokens here
...
13 |     b!();
   |     ----- in this macro invocation

error: aborting due to 2 previous errors

error: Could not compile `b`.

To learn more, run the command again with --verbose.
```
</details>
The output is the same as today, except for an addition to the note which aids discoverability of the new environment variable.

<br/><br/>
Running _with_ env var:
<details>

```
$ RUST_MACRO_BACKTRACE=1 cargo +custom run
   Compiling b v0.1.0
error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `error`
 --> <a macros>:1:72
  |
1 | (  ) => { a ! ( @ ) } ; ( @ ) => { a ! ( @ @ ) } ; ( @ @ ) => { syntax error ;
  |                                                                       -^^^^^ unexpected token
  |                                                                       |
  |                                                                       expected one of 8 possible tokens here
src/main.rs:12:5: 12:10 note: in this expansion of a! (defined in <a macros>)
<a macros>:1:11: 1:20 note: in this expansion of a! (defined in <a macros>)
<a macros>:1:36: 1:47 note: in this expansion of a! (defined in <a macros>)

error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `error`
 --> src/main.rs:7:16
  |
7 |         syntax error;
  |               -^^^^^ unexpected token
  |               |
  |               expected one of 8 possible tokens here
src/main.rs:12:5: 12:10 note: in this expansion of a! (defined in <a macros>)
<a macros>:1:11: 1:20 note: in this expansion of a! (defined in <a macros>)
<a macros>:1:36: 1:47 note: in this expansion of a! (defined in <a macros>)

error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `error`
 --> src/main.rs:7:16
  |
7 |         syntax error;
  |               -^^^^^ unexpected token
  |               |
  |               expected one of 8 possible tokens here
src/main.rs:13:5: 13:10 note: in this expansion of b! (defined in src/main.rs)
src/main.rs:4:13: 4:18 note: in this expansion of b! (defined in src/main.rs)
src/main.rs:5:14: 5:20 note: in this expansion of b! (defined in src/main.rs)

error: aborting due to 2 previous errors

error: Could not compile `b`.

To learn more, run the command again with --verbose.
```
</details>

The output is hard to read, but better than nothing (and it's exactly what we used to have before the infamous `fix_multispans_in_std_macros`).

<br/><br/>
Wishlist:

- Save the actual source of macros in crate metadata, not just AST, so the output can be improved
    - Hopefully this would allow line numbers in the trace as well
- Show the actual macro invocations in the traces

r? @nrc

src/librustdoc/test.rs

@@ -81,7 +81,9 @@ pub fn run(input: &str,
 
     let codemap = Rc::new(CodeMap::new(sessopts.file_path_mapping()));
     let handler =
-        errors::Handler::with_tty_emitter(ColorConfig::Auto, true, false, Some(codemap.clone()));
+        errors::Handler::with_tty_emitter(ColorConfig::Auto,
+                                          true, false,
+                                          Some(codemap.clone()));
 
     let cstore = Rc::new(CStore::new(box rustc_trans::LlvmMetadataLoader));
     let mut sess = session::build_session_(