Extend the json compiler output to produce simulatneously the short and long type output without writing to disk

rustc currently will trim long type names by default, and write the full path that got elided to a file on disk (generally on the target directory). This rarely causes any trouble and manages to keep terminal output legible even in the face of terribly long types. But there are cases where people get alarmed by the existence of these files (like recently at https://www.reddit.com/r/rust/comments/1ps82dl/media_what_is_this_longtypetxt_thing/), and I suppose these people might be interacting with rustc through VSCode and rust-analyzer, hence rarely seeing the full diagnostic output. If the json output had both the short and long output without ever printing the file to disk, interactive tools like rust-analyzer can show the shorter output by default with a button to expand each diagnostic, without ever polluting the user's drive, at the cost of the occasional duplicated diagnostic information in the json output.

Start by tracing rustc's JSON diagnostic generation and the existing handling of trimmed long type names and long-type files. Determine how both short and full output could be represented without writing the full path to disk, then verify that interactive consumers can select the shorter or expanded diagnostic output.