rustdoc-json: Precise capturing JSON is untyped

[Enselic]

Given this function:

pub fn capture<'a, T>(x: &'a (), y: T) -> impl Sized + use<'a, T> {
    (x, y)
}

we can see this rustdoc JSON for the function output type:

rustdoc +nightly -Zunstable-options --output-format json src/lib.rs

  "output": {
      "impl_trait": [
          {
              "trait_bound": {
                  "trait": {
                      "path": "Sized",
                      "id": 1,
                      "args": {
                          "angle_bracketed": {
                              "args": [],
                              "constraints": []
                          }
                      }
                  },
                  "generic_params": [],
                  "modifier": "none"
              }
          },
          {
              "use": [
                  "'a",
                  "T"
              ]
          }
      ]
  },

Note that the use data is completely untyped and just raw strings:

          {
              "use": [
                  "'a",
                  "T"
              ]
          }

We probably want to use different JSON types at least for generic args and lifetimes before we stabilize the format.

Right now cargo-public-api resorts to a "if string begins with ' " hack.

[aDotInTheVoid]

By the time it's in clean, it's already a Vec<Symbol>: https://doc.rust-lang.org/nightly/nightly-rustc/rustdoc/clean/types/enum.GenericBound.html#variant.Use

But the distinction does exist in Hir: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/enum.GenericBound.html

So a fix to this would need to:

1. Change rustdoc::clean::types::GenericBound::Use to use PreciseCapturingArg (either the HIR one, or a new one that's just in clean)
   1.1. Fix fallout from this in rustdoc::html
2. Change rustdoc_json_types::GenericBound::Use to use PreciseCapturingArg (which must be a new enum in rustdoc_json_types)
3. Update tests/rustdoc-json (and potentially add new ones if there's not enough coverage).
4. Bump rustdoc_json_types::FORMAT_VERSION

[Kohei316]

@rustbot claim