[clang] [clang][Modules] Adding Specific Diagnostics when a Finalized PCM is Out-of-date due to Input File Changes (PR #209857)

[Cyndy Ishida via cfe-commits]

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@@ -23,6 +23,10 @@ def err_fe_ast_file_modified : Error<
     DefaultFatal;
 def note_fe_ast_file_modified : Note<
     "%select{size|mtime|content}0 changed%select{| from expected %2 to %3}1">;
+def note_fe_ast_file_modified_finalized : Note<
+    "module '%0' was already finalized in this compilation and so cannot be "
+    "rebuilt; this is usually caused by an input changing after the module was "
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cyndyishida wrote:

nit: i'd change  "after the module was built" to something more vague like "from misconfiguration"

https://github.com/llvm/llvm-project/pull/209857