[clang] [C++20][Modules] Implement P1857R3 Modules Dependency Discovery (PR #107168)

[via cfe-commits]

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@@ -986,6 +989,14 @@ def warn_module_conflict : Warning<
   InGroup<ModuleConflict>;
 
 // C++20 modules
+def err_pp_module_name_is_macro : Error<
+  "%select{module|partition}0 name component %1 cannot be a object-like macro">;
+def err_pp_module_expected_ident : Error<
+  "expected %select{identifier after '.' in |}0module name">;
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yronglin wrote:

SGTM! Since we have alread implement CWG2947 in this patch, I have a question about the following case:
```cpp
// #define m(x) x
export module m
     (foo);
```
Should we emit an diagnostic the `(` in line 2 in phase 7? We can't seen it in the handling of module directive in phase 4.

https://github.com/llvm/llvm-project/pull/107168