Split rustc_hir_id out of rustc_hir

Some crates depend on `rustc_hir` but only want `HirId` and similar id
types. `rustc_hir` is a heavy dependency, since it pulls in
`rustc_target`. Split these types out into their own crate
`rustc_hir_id`.

This allows `rustc_errors` to drop its direct dependency on `rustc_hir`.

(`rustc_errors` still depends on `rustc_hir` indirectly through
`rustc_lint_defs`; a subsequent commit will fix that.)

compiler/rustc_errors/src/lib.rs

@@ -60,7 +60,7 @@ pub use rustc_error_messages::{
     fallback_fluent_bundle, fluent_bundle, into_diag_arg_using_display,
 };
 use rustc_hashes::Hash128;
-use rustc_hir::HirId;
+use rustc_hir_id::HirId;
 pub use rustc_lint_defs::{Applicability, listify, pluralize};
 use rustc_lint_defs::{Lint, LintExpectationId};
 use rustc_macros::{Decodable, Encodable};