[PatternRewriter] Disable copy/assign operators.

We had a [bad bug](https://github.com/llvm/circt/commit/69655864ee38167016506e9dae2eb1eb43dc3ba5) 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