Auto merge of #35702 - jonathandturner:add_backtrace_labels, r=nikomatsakis

Auto merge of #35702 - jonathandturner:add_backtrace_labels, r=nikomatsakis

Replace macro backtraces with labeled local uses

This PR (which builds on https://github.com/rust-lang/rust/pull/35688) follows from the conversations on how best to [handle the macro backtraces](https://internals.rust-lang.org/t/improving-macro-errors/3809).  The feeling there was that there were two different "groups" of users.

The first group, the macro users, rarely (and likely never) want to see the macro backtrace.  This is often more confusing to users as it will be talking about code they didn't write.

The second group, the macro writers, are trying to debug a macro.  They'll want to see something of the backtrace so that they can see where it's going wrong and what the steps were to get there.

For the first group, it seems clear that we don't want to show *any* macro backtrace.  For the second group, we want to show enough to help the macro writer.

This PR uses a heuristic.  It will only show any backtrace steps if they are in the same crate that is being compiled.  This keeps errors in foreign crates from showing to users that didn't need them.

Additionally, in asking around I repeated heard that the middle steps of the backtrace are rarely, if ever, used in practice.  This PR takes and applies this knowledge.  Now, instead of a full backtrace, the user is given the error underline inside of a local macro as well as the use site as a secondary label.  This effectively means seeing the root of the error and the top of the backtrace, eliding the middle steps.

Rather than being the "perfect solution", this PR opts to take an incremental step towards a better experience.  Likely it would help to have additional macro debugging tools, as they could be much more verbose than we'd likely want to use in the error messages themselves.

Some examples follow.

**Example 1**

Before:

<img width="1275" alt="screen shot 2016-08-15 at 4 13 18 pm" src="https://cloud.githubusercontent.com/assets/547158/17682828/3948cea2-6303-11e6-93b4-b567e9d62848.png">

After:

<img width="596" alt="screen shot 2016-08-15 at 4 13 03 pm" src="https://cloud.githubusercontent.com/assets/547158/17682832/3d670d8c-6303-11e6-9bdc-f30a30bf11ac.png">

**Example 2**

Before:

<img width="918" alt="screen shot 2016-08-15 at 4 14 35 pm" src="https://cloud.githubusercontent.com/assets/547158/17682870/722225de-6303-11e6-9175-336a3f7ce308.png">

After:

<img width="483" alt="screen shot 2016-08-15 at 4 15 01 pm" src="https://cloud.githubusercontent.com/assets/547158/17682872/7582cf6c-6303-11e6-9235-f67960f6bd4c.png">