This PR adds a program which uses the JSON output from #24884 to generate a webpage with descriptions of each diagnostic error.
The page is constructed by hand, with calls to `rustdoc`'s markdown renderers where needed. I opted to generate HTML directly as I think it's more flexible than generating a markdown file and feeding it into the `rustdoc` executable. I envision adding the ability to filter errors by their properties (description, no description, used, unused), which is infeasible using the whole-file markdown approach due to the need to wrap each error in a `<div>` (markdown inside tags isn't rendered).
Architecturally, I wasn't sure how to add this generator to the distribution. For the moment I've settled on a separate Rust program in `src/etc/` that gets compiled and run by a custom makefile rule. This approach doesn't seem too hackish, but I'm unsure if my usage of makefile variables is correct, particularly the call to `rustc` (and the `LD_LIBRARY_PATH` weirdness). Other options I considered were:
* Integrate the error-index generator into `rustdoc` so that it gets invoked via a flag and can be built as part of `rustdoc`.
* Add the error-index-generator as a "tool" to the `TOOLS` array, and make use of the facilities for building tools. The main reason I didn't do this was because it seemed like I'd need to add lots of stuff. I'm happy to investigate this further if it's the preferred method.
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