[clang] [Modules][Diagnostic] Improve diagnostics for stale module dependencies (PR #167713)

[Chuanqi Xu via cfe-commits]

================
@@ -199,7 +199,10 @@ class ModuleManager {
     Missing,
 
     /// The module file is out-of-date.
-    OutOfDate
+    OutOfDate,
+
+    /// The importer file is out-of-date
+    ImporterOutOfDate
----------------
ChuanqiXu9 wrote:

Other parts looks good.  But the name ImporterOutOfDate  looks confusing. How can the importer be out of date?

https://github.com/llvm/llvm-project/pull/167713