Merge #6645

6645: Publish diagnostics for macro expansion errors r=matklad a=jonas-schievink

This adds 2 new diagnostics, emitted during name resolution:

* `unresolved-proc-macro`, a weak warning that is emitted when a proc macro is supposed to be expanded, but was not provided by the build system. This usually means that proc macro support is turned off, but may also indicate setup issues when using rust-project.json. Being a weak warning, this should help set expectations when users see it, while not being too obstructive. We do not yet emit this for attribute macros though, just custom derives and `!` macros.
* `macro-error`, which is emitted when any macro (procedural or `macro_rules!`) fails to expand due to some error. This is an error-level diagnostic, but currently still marked as experimental, because there might be spurious errors and this hasn't been tested too well.

This does not yet emit diagnostics when expansion in item bodies fails, just for module-level macros.

Known bug: The "proc macro not found" diagnostic points at the whole item for custom derives, it should just point at the macro's name in the `#[derive]` list, but I haven't found an easy way to do that.

Screenshots:

![screenshot-2020-11-26-19:54:14](https://user-images.githubusercontent.com/1786438/100385782-f8bc2300-3023-11eb-9f27-e8f8ce9d6114.png)
![screenshot-2020-11-26-19:55:39](https://user-images.githubusercontent.com/1786438/100385784-f954b980-3023-11eb-9617-ac2eb0a0a9dc.png)

Co-authored-by: Jonas Schievink <jonasschievink@gmail.com>

diff --cc crates/hir_expand/src/db.rs
Simple merge