feature: Support navigation on primitives

[davidbarsky]

We could maybe point the primitive types to their core re-exports https://doc.rust-lang.org/src/core/primitive.rs.html or rustdoc module shims https://doc.rust-lang.org/src/core/primitive_docs.rs.html

Originally posted by @Veykril in #19105 (comment)

[Wilfred]

FWIW we do see users sometimes doing go-to-def on primitive types, so taking them somewhere would be a nice experience for them. E.g. https://doc.rust-lang.org/src/core/primitive_docs.rs.html#1413 does seem nice for u64.

[Veykril]

Fwiw we already have logic for hover to resolve primitives to those modules for doc rendering purposes

[ChayimFriedman2]

FWIW I don't agree this is useful. Goto def should give me more information about the thing, but here there isn't really anything new (besides the docs, and we already show them on hover).

[Wilfred]

Users do generally expect that they can go-to-def on things that have hover information. There is some additional information they can learn from viewing the 'definition', e.g.:

#[rustc_doc_primitive = "i128"]
//
/// The 128-bit signed integer type.
#[stable(feature = "i128", since = "1.26.0")]
mod prim_i128 {}

I can see that it's a primitive, and when it was introduced to rust.