fix(compiler): fix a few non-tree-shakeable code patterns (#24677)

This change makes @angular/compiler more tree-shakeable by changing
an enum to a const enum and by getting rid of a top-level map that
the tree-shaker was seeing as a reference which caused r3_identifiers
to be retained.

This drops a few hundred bytes of JS from tree-shaken ngtsc compiled
apps.

PR Close #24677

packages/core/test/bundling/hello_world_r2/bundle.golden_symbols.json

@@ -356,9 +356,6 @@
   {
     "name": "CORE"
   },
-  {
-    "name": "CORE$1"
-  },
   {
     "name": "CR"
   },
@@ -803,9 +800,6 @@
   {
     "name": "Identifiers"
   },
-  {
-    "name": "Identifiers$1"
-  },
   {
     "name": "IfStmt"
   },
@@ -1208,9 +1202,6 @@
   {
     "name": "PreparsedElementType"
   },
-  {
-    "name": "PropertyBindingType"
-  },
   {
     "name": "PropertyRead"
   },