[mlir] Partially revert #162903 (#164464)

With #163862, this is not really necessary and causes downstream issues.

Author: jansvoboda11

mlir/lib/IR/Diagnostics.cpp

@@ -378,10 +378,8 @@ struct SourceMgrDiagnosticHandlerImpl {
     }
 
     // Otherwise, try to load the source file.
-    auto bufferOrErr = llvm::MemoryBuffer::getFile(filename);
-    if (!bufferOrErr)
-      return 0;
-    unsigned id = mgr.AddNewSourceBuffer(std::move(*bufferOrErr), SMLoc());
+    std::string ignored;
+    unsigned id = mgr.AddIncludeFile(std::string(filename), SMLoc(), ignored);
     filenameToBufId[filename] = id;
     return id;
   }