Merge `TypeMapper` back into `Importer` - part 3/N: `Insert`.

This CL is one of steps toward merging `TypeMapper` back into
`Importer`.  The merge is:
- possible (after https://github.com/google/crubit/commit/16a5610d96eda5720bb57b5f927b00f00635ddf0 which means that there is only 1 instance
  of `TypeMapper`, because `ImportFields` doesn't anymore need to
  temporarily assume that the containing record can be imported),
- desirable (to support b/228868369 we need to let `ConvertType` call
  into `Importer::ConvertTemplateSpecializationType`).

This CL makes `TypeMapper::Insert` a private method of `TypeMapper`.
Callers of `TypeMapper::Insert` are transitioned to call the newly
introduced, equivalent `Importer::MarkAsSuccessfullyImported`.

PiperOrigin-RevId: 451509535
diff --git a/rs_bindings_from_cc/importer.cc b/rs_bindings_from_cc/importer.cc
index 2043e16..3d193fd 100644
--- a/rs_bindings_from_cc/importer.cc
+++ b/rs_bindings_from_cc/importer.cc
@@ -885,6 +885,10 @@
   }
 }
 
+void Importer::MarkAsSuccessfullyImported(const clang::TypeDecl* decl) {
+  type_mapper_.Insert(decl);
+}
+
 bool Importer::HasBeenAlreadySuccessfullyImported(
     const clang::TypeDecl* decl) const {
   return type_mapper_.Contains(decl);