Auto merge of #17799 - Veykril:syntax-bridge, r=Veykril

Split out syntax-bridge into a separate crate

This functionality is not really tied to mbe macros, so imo it has no place in that crate.

crates/span/src/hygiene.rs

@@ -79,6 +79,10 @@ impl SyntaxContextId {
 #[derive(Copy, Clone, Hash, PartialEq, Eq)]
 pub struct SyntaxContextData {
     /// Invariant: Only [`SyntaxContextId::ROOT`] has a [`None`] outer expansion.
+    // FIXME: The None case needs to encode the context crate id. We can encode that as the MSB of
+    // MacroCallId is reserved anyways so we can do bit tagging here just fine.
+    // The bigger issue is that that will cause interning to now create completely separate chains
+    // per crate. Though that is likely not a problem as `MacroCallId`s are already crate calling dependent.
     pub outer_expn: Option<MacroCallId>,
     pub outer_transparency: Transparency,
     pub parent: SyntaxContextId,