[mlir][bufferize][NFC] Move FuncOp bufferization to BufferizableOpInterface impl

FuncOps are now less special. They must still be analyzed + bufferized in a certain order, but they are now bufferized same as other ops that have a region: Bufferize the op first (`bufferize` interface method), then bufferize the region body with other bufferization patterns. In the case of FuncOps, the function signature is bufferized together with ReturnOps. Similar to how, e.g., scf.for ops are bufferized together with scf.yield ops.

This change is essentially a reimplementation of the FuncOp bufferization, but mostly NFC from a user's perspective (apart from error messages). This change is in preparation of moving the code to the bufferization dialect.

Differential Revision: https://reviews.llvm.org/D123214