[PatternRewriter] Disable copy/assign operators.

We had a [bad bug](69655864ee) over in CIRCT
caused by accidentally passing around PatternRewriter
by value.  There is no reason to support copy/assignment
of the pattern rewriter, so disable it.

Differential Revision: https://reviews.llvm.org/D107232

mlir/include/mlir/IR/PatternMatch.h

@@ -836,6 +836,9 @@ protected:
   }
 
 private:
+  void operator=(const RewriterBase &) = delete;
+  RewriterBase(const RewriterBase &) = delete;
+
   /// 'op' and 'newOp' are known to have the same number of results, replace the
   /// uses of op with uses of newOp.
   void replaceOpWithResultsOfAnotherOp(Operation *op, Operation *newOp);