[clang] [C++20] [Modules] Warn for importing implementation partition unit in interface units (PR #108493)

[via cfe-commits]

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@@ -439,6 +439,9 @@ def warn_deprecated_literal_operator_id: Warning<
   "is deprecated">, InGroup<DeprecatedLiteralOperator>, DefaultIgnore;
 def warn_reserved_module_name : Warning<
   "%0 is a reserved name for a module">, InGroup<ReservedModuleIdentifier>;
+def warn_import_implementation_partition_unit_in_interface_unit : Warning<
+  "it is not suggested to import implementation partition unit in interface unit">,
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cor3ntin wrote:

```suggestion
  "importing an implementation partition unit in a module interface is not recommended. Symbols exported by %0 may not be reachable">,
```
Maybe something like that ?

https://github.com/llvm/llvm-project/pull/108493