…matsakis

rustc_typeck: construct {Closure,Generator}Substs more directly.

We've previously not had a way to create `{Closure,Generator}Substs` other than instantiating all generics as inference variables and unifying the inference types (extracted using the regular `{Closure,Generator}Substs` accessors), with the actual types.

With this PR, those hacks, and assumptions about the order of closure/generator-specific components, are replaced with a simple API where the base `Substs` are combined with the additional information into a `{Closure,Generator}Substs`.
This might also be faster than relying inference, although probably not by much.

r? @nikomatsakis cc rust-lang#53488 @blitzerr

Rollup of 10 pull requests

Successful merges:

 - rust-lang#74204 (Don't visit foreign function bodies when lowering ast to hir)
 - rust-lang#74314 (rustc_typeck: construct {Closure,Generator}Substs more directly.)
 - rust-lang#74346 (Use LocalDefId instead of HirId for reachable_set elements.)
 - rust-lang#74399 (Move DelaySpanBugEmitted to ty::context)
 - rust-lang#75177 (Add regression test for issue-66768)
 - rust-lang#75223 (Add #[track_caller] to `Session::delay_span_bug`)
 - rust-lang#75423 (Move to intra-doc links for /library/core/src/hint.rs)
 - rust-lang#75485 (pin docs: add some forward references)
 - rust-lang#75569 (Bump minor version of emsdk to 1.38.47)
 - rust-lang#75596 (Switch to intra-doc links in /sys/windows/ext/{ffi,fs,process}.rs)

Failed merges:

r? @ghost