Auto merge of #79539 - aDotInTheVoid:json-mvp, r=jyn514

Rustdoc: JSON backend experimental impl, with new tests.

Based on #75114 by `@P1n3appl3`

The first commit is all of #75114, but squased to 1 commit, as that was much easier to rebase onto master.

The git history is a mess, but I think I'll edit it after review, so it's obvious whats new.

## Still to do

- [ ] Update docs.
- [ ] Add bless option to tests.
- [ ] Add test option for multiple files in same crate.
- [ ] Decide if the tests should check for json to be equal or subset.
- [ ] Go through the rest of the review for the original pr. (This is open because the test system is done(ish), but stuff like [not using a hashmap](https://github.com/rust-lang/rust/pull/75114#discussion_r519474420) and [using `CRATE_DEF_INDEX` ](https://github.com/rust-lang/rust/pull/75114#discussion_r519470764) hasn't)

I'm also sure how many of these we need to do before landing on nightly, as it would be nice to get this in tree, so it isn't effected by churn like #79125, #79041, #79061

r? `@jyn514`

1 files changed, 456 insertions, 0 deletions

diff --git a/src/test/rustdoc-json/structs.expected b/src/test/rustdoc-json/structs.expected
new file mode 100644
index 00000000000..799829de3fd
--- /dev/null
+++ b/src/test/rustdoc-json/structs.expected
@@ -0,0 +1,456 @@
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