[mlir][PDL] Define a new PDLInterp::FuncOp operation and drop uses of FuncOp

[mlir][PDL] Define a new PDLInterp::FuncOp operation and drop uses of FuncOp

Defining our own function operation allows for the PDL interpreter
to be more self contained, and also removes any dependency on FuncOp;
which is moving out of the Builtin dialect.

Differential Revision: https://reviews.llvm.org/D121253