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

[Aaron Ballman via cfe-commits]

================
@@ -439,6 +439,10 @@ 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<
+  "importing an implementation partition unit in a module interface is not recommended. "
+  "Names in by %0 may not be reachable">,
----------------
AaronBallman wrote:

```suggestion
  "importing an implementation partition unit in a module interface is not recommended; "
  "names from '%0' may not be reachable">,
```

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