[mlir] Apply ClangTidy finding (NFC)

prefer using 'override' or (rarely) 'final' instead of 'virtual'
diff --git a/mlir/lib/Conversion/MPIToLLVM/MPIToLLVM.cpp b/mlir/lib/Conversion/MPIToLLVM/MPIToLLVM.cpp
index 084a60e..aa7ed81 100644
--- a/mlir/lib/Conversion/MPIToLLVM/MPIToLLVM.cpp
+++ b/mlir/lib/Conversion/MPIToLLVM/MPIToLLVM.cpp
@@ -62,7 +62,7 @@
 
   explicit MPIImplTraits(ModuleOp &moduleOp) : moduleOp(moduleOp) {}
 
-  virtual ~MPIImplTraits() = default;
+  ~MPIImplTraits() override = default;
 
   ModuleOp &getModuleOp() { return moduleOp; }
 
@@ -98,7 +98,7 @@
 public:
   using MPIImplTraits::MPIImplTraits;
 
-  virtual ~MPICHImplTraits() = default;
+  ~MPICHImplTraits() override = default;
 
   Value getCommWorld(const Location loc,
                      ConversionPatternRewriter &rewriter) override {