Rename Record::decl_id to id, Func::decl_id to record_decl_id.

The intention is to meke it clearer which items have a decl_id (record), and which items reference other items (func, mapped types).

While doing this I realized Func had a decl_id similarly to record, but I believe the intention was for methods to be able to reference their owning record (at least I think Devin is planning to rely on that AFAIK). So I fixed that. It is possible that in the future we will add Func::id field as well.

PiperOrigin-RevId: 413751180
diff --git a/rs_bindings_from_cc/src_code_gen.rs b/rs_bindings_from_cc/src_code_gen.rs
index 0cae54e..2792bcc 100644
--- a/rs_bindings_from_cc/src_code_gen.rs
+++ b/rs_bindings_from_cc/src_code_gen.rs
@@ -653,9 +653,9 @@
     // `ir_testing`.
     fn test_duplicate_decl_ids_err() {
         let mut r1 = ir_record("R1");
-        r1.decl_id = DeclId(42);
+        r1.id = DeclId(42);
         let mut r2 = ir_record("R2");
-        r2.decl_id = DeclId(42);
+        r2.id = DeclId(42);
         let result = make_ir_from_items([r1.into(), r2.into()]);
         assert!(result.is_err());
         assert!(result.unwrap_err().to_string().contains("Duplicate decl_id found in"));
@@ -665,7 +665,7 @@
     fn test_simple_function() -> Result<()> {
         let ir = make_ir_from_items([Item::Func(Func {
             name: UnqualifiedIdentifier::Identifier(ir_id("add")),
-            decl_id: DeclId(42),
+            record_decl_id: None,
             owning_target: "//foo:bar".into(),
             mangled_name: "_Z3Addii".to_string(),
             doc_comment: None,
@@ -706,7 +706,7 @@
         let ir = make_ir_from_parts(
             vec![Item::Func(Func {
                 name: UnqualifiedIdentifier::Identifier(ir_id("add")),
-                decl_id: DeclId(42),
+                record_decl_id: None,
                 owning_target: "//foo:bar".into(),
                 mangled_name: "_Z3Addii".to_string(),
                 doc_comment: None,
@@ -761,7 +761,7 @@
     fn test_simple_struct() -> Result<()> {
         let ir = make_ir_from_items([Item::Record(Record {
             identifier: ir_id("SomeStruct"),
-            decl_id: DeclId(42),
+            id: DeclId(42),
             owning_target: "//foo:bar".into(),
             doc_comment: None,
             fields: vec![
@@ -881,7 +881,7 @@
     fn test_ptr_func() -> Result<()> {
         let ir = make_ir_from_items([Item::Func(Func {
             name: UnqualifiedIdentifier::Identifier(Identifier { identifier: "Deref".to_string() }),
-            decl_id: DeclId(42),
+            record_decl_id: None,
             owning_target: "//foo:bar".into(),
             mangled_name: "_Z5DerefPKPi".to_string(),
             doc_comment: None,
@@ -1008,7 +1008,7 @@
     fn test_doc_comment_func() -> Result<()> {
         let ir = make_ir_from_items([Item::Func(Func {
             name: UnqualifiedIdentifier::Identifier(ir_id("func")),
-            decl_id: DeclId(42),
+            record_decl_id: None,
             owning_target: "//foo:bar".into(),
             mangled_name: "foo".to_string(),
             doc_comment: Some("Doc Comment".to_string()),
@@ -1030,7 +1030,7 @@
     fn test_doc_comment_record() -> Result<()> {
         let ir = make_ir_from_items([Item::Record(Record {
             identifier: ir_id("SomeStruct"),
-            decl_id: DeclId(42),
+            id: DeclId(42),
             owning_target: "//foo:bar".into(),
             doc_comment: Some("Doc Comment\n\n * with bullet".to_string()),
             alignment: 0,