Auto merge of #75145 - davidtwco:issue-60607-preallocate-defid-for-lang-items, r=petrochenkov

Reference lang items during AST lowering

Fixes #60607 and fixes #61019.

This PR introduces `QPath::LangItem` to the HIR and uses it in AST lowering instead of constructing a `hir::Path` from a slice of symbols:

- Credit for much of this work goes to @matthewjasper, I basically just [rebased their earlier work](matthewjasper@a227c70#diff-c0f791ead38d2d02916faaad0f56f41d).
- ~~Changes to Clippy might not be correct, they compile but attempting to run tests through `./x.py` produced failures which appeared spurious, so I didn't run any clippy tests.~~
- Changes to save analysis might not be correct - tests pass but I don't have a lot of confidence in those changes being correct.
- I've used `GenericBounds::LangItemTrait` rather than changing `PolyTraitRef`, as suggested by @matthewjasper [in this comment](matthewjasper@a227c70#r40107992) but I'd prefer that be left for a follow-up.
- I've split things into smaller commits fairly arbitrarily to make the diff easier to review, each commit should compile but might not pass tests until the final commit.

r? @oli-obk
cc @matthewjasper

Author: bors

library/core/src/convert/mod.rs

@@ -385,6 +385,7 @@ pub trait Into<T>: Sized {
 ))]
 pub trait From<T>: Sized {
     /// Performs the conversion.
+    #[cfg_attr(not(bootstrap), lang = "from")]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn from(_: T) -> Self;
 }

library/core/src/future/future.rs

@@ -96,6 +96,7 @@ pub trait Future {
     /// [`Context`]: ../task/struct.Context.html
     /// [`Waker`]: ../task/struct.Waker.html
     /// [`Waker::wake`]: ../task/struct.Waker.html#method.wake
+    #[cfg_attr(not(bootstrap), lang = "poll")]
     #[stable(feature = "futures_api", since = "1.36.0")]
     fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>;
 }