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

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@@ -1775,10 +1775,18 @@ def ext_bit_int : Extension<
 } // end of Parse Issue category.
 
 let CategoryName = "Modules Issue" in {
-def err_unexpected_module_decl : Error<
-  "module declaration can only appear at the top level">;
+def err_invalid_module_or_import_directive : Error<
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yronglin wrote:

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

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