[mlir] Fix a warning

This patch fixes:

  mlir/include/mlir/Analysis/AliasAnalysis/LocalAliasAnalysis.h:24:7:
  error: 'mlir::LocalAliasAnalysis' has virtual functions but
  non-virtual destructor [-Werror,-Wnon-virtual-dtor]

mlir/include/mlir/Analysis/AliasAnalysis/LocalAliasAnalysis.h

@@ -23,6 +23,8 @@ namespace mlir {
 /// checks to see if they alias.
 class LocalAliasAnalysis {
 public:
+  virtual ~LocalAliasAnalysis() = default;
+
   /// Given two values, return their aliasing behavior.
   AliasResult alias(Value lhs, Value rhs);