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

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@@ -986,6 +989,10 @@ def warn_module_conflict : Warning<
   InGroup<ModuleConflict>;
 
 // C++20 modules
+def err_module_decl_in_header
+    : Error<"module declaration must not come from an #include directive">;
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yronglin wrote:

Added test clang/test/CXX/module/cpp.pre/p1.cpp

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