[NFC] [clangd] [C++20 Modules] Add a warning if clangd detected multiple

source declares the same module

Now clangd assumes no duplicated module declared by different source
file in a sinlge project. But in practice, it may not be the case.

Although we can't fix it now, emitting a warning is helpful for users to
understand what's going on.

Author: ChuanqiXu9

clang-tools-extra/clangd/ScanningProjectModules.cpp

@@ -122,8 +122,17 @@ ModuleDependencyScanner::scan(PathRef FilePath,
   ModuleDependencyInfo Result;
 
   if (ScanningResult->Provides) {
-    ModuleNameToSource[ScanningResult->Provides->ModuleName] = FilePath;
     Result.ModuleName = ScanningResult->Provides->ModuleName;
+
+    auto [Iter, Inserted] = ModuleNameToSource.try_emplace(
+        ScanningResult->Provides->ModuleName, FilePath);
+
+    if (!Inserted && Iter->second != FilePath) {
+      elog("Detected multiple source files ({0}, {1}) declaring the same "
+           "module: '{2}'. "
+           "Now clangd may find the wrong source in such case.",
+           Iter->second, FilePath, ScanningResult->Provides->ModuleName);
+    }
   }
 
   for (auto &Required : ScanningResult->Requires)