[mlir][NFC] Provide accessor for TableGen record for constraints

[math-fehr]

Summary

Besides accessing the record, there is currently no way to access all possible
constraint informations, such as the base constraint of a variadic constraint
for example.

[math-fehr]

Thanks, would it be possible to land it for me?

mlir/include/mlir/TableGen/Constraint.h

public:
 
   /// Get an opaque pointer to the constraint.
   const void *getAsOpaquePointer() const { return def; }
   /// Construct a constraint from the opaque pointer representation.
   static Constraint getFromOpaquePointer(const void *ptr) {
     return Constraint(reinterpret_cast<const llvm::Record *>(ptr));
   }
 
+  // Return the underlying def.
+  const llvm::Record *getDef() const { return def; }
+
 protected:
   Constraint(Kind kind, const llvm::Record *record);
 
   // The TableGen definition of this constraint.
   const llvm::Record *def;
 
 private:
   // What kind of constraint this is.