[MLIR] Move SymbolOpInterface::isPublic() and friends to SymbolOpInterface Trait.

- This will allow calling these functions from Op's that support this interface (like FuncOp) directly:

```
FuncOp func = ...
 func.isPrivate()
```

Differential Revision: https://reviews.llvm.org/D82060

mlir/include/mlir/IR/SymbolInterfaces.td

@@ -58,6 +58,24 @@ def Symbol : OpInterface<"SymbolOpInterface"> {
         mlir::SymbolTable::setSymbolVisibility(this->getOperation(), vis);
       }]
     >,
+    InterfaceMethod<"Returns true if this symbol has nested visibility.",
+      "bool", "isNested", (ins),  [{}],
+      /*defaultImplementation=*/[{
+        return getVisibility() == mlir::SymbolTable::Visibility::Nested;
+      }]
+    >,
+    InterfaceMethod<"Returns true if this symbol has private visibility.",
+      "bool", "isPrivate", (ins),  [{}],
+      /*defaultImplementation=*/[{
+        return getVisibility() == mlir::SymbolTable::Visibility::Private;
+      }]
+    >,
+    InterfaceMethod<"Returns true if this symbol has public visibility.",
+      "bool", "isPublic", (ins),  [{}],
+      /*defaultImplementation=*/[{
+        return getVisibility() == mlir::SymbolTable::Visibility::Public;
+      }]
+    >,
     InterfaceMethod<[{
         Get all of the uses of the current symbol that are nested within the
         given operation 'from'.
@@ -124,20 +142,11 @@ def Symbol : OpInterface<"SymbolOpInterface"> {
   }];
 
   let extraClassDeclaration = [{
-    using Visibility = mlir::SymbolTable::Visibility;
-
     /// Custom classof that handles the case where the symbol is optional.
     static bool classof(Operation *op) {
       return Base::classof(op)
         && op->getAttr(::mlir::SymbolTable::getSymbolAttrName());
     }
-
-    /// Returns true if this symbol has nested visibility.
-    bool isNested() { return getVisibility() == Visibility::Nested; }
-    /// Returns true if this symbol has private visibility.
-    bool isPrivate() { return getVisibility() == Visibility::Private; }
-    /// Returns true if this symbol has public visibility.
-    bool isPublic() { return getVisibility() == Visibility::Public; }
   }];
 
   let extraTraitClassDeclaration = [{