[mlir] Apply ClangTidyLegacy findings.

Use '= default' to define a trivial default constructor.

mlir/include/mlir/IR/Attributes.h

@@ -34,7 +34,7 @@ public:
   using ValueType = void;
   using AbstractTy = AbstractAttribute;
 
-  constexpr Attribute() {}
+  constexpr Attribute() = default;
   /* implicit */ Attribute(const ImplType *impl)
       : impl(const_cast<ImplType *>(impl)) {}
 

mlir/include/mlir/IR/IntegerSet.h

@@ -45,7 +45,7 @@ class IntegerSet {
 public:
   using ImplType = detail::IntegerSetStorage;
 
-  constexpr IntegerSet() {}
+  constexpr IntegerSet() = default;
   explicit IntegerSet(ImplType *set) : set(set) {}
 
   static IntegerSet get(unsigned dimCount, unsigned symbolCount,

mlir/include/mlir/IR/OpImplementation.h

@@ -215,7 +215,7 @@ public:
 protected:
   /// Initialize the printer with no internal implementation. In this case, all
   /// virtual methods of this class must be overriden.
-  AsmPrinter() {}
+  AsmPrinter() = default;
 
 private:
   AsmPrinter(const AsmPrinter &) = delete;

mlir/include/mlir/IR/Types.h

@@ -83,7 +83,7 @@ public:
 
   using AbstractTy = AbstractType;
 
-  constexpr Type() {}
+  constexpr Type() = default;
   /* implicit */ Type(const ImplType *impl)
       : impl(const_cast<ImplType *>(impl)) {}