Auto merge of #6081 - dtolnay:raw, r=alexcrichton

Fix missing messages when --message-format=json is deeply nested

This commit switches from `serde_json::Value` to [`RawValue`](https://docs.rs/serde_json/1.0/serde_json/value/struct.RawValue.html), which can process arbitrarily deeply nested JSON content without recursion.

Fixes #5992. @ehuss