[mlir] Add func-bufferize pass.

This is the most basic possible finalizing bufferization pass, which I
also think is sufficient for most new use cases. The more concentrated
nature of this pass also greatly clarifies the invariants that it
requires on its input to safely transform the program (see the
pass description in Passes.td).

With this pass, I have now upstreamed practically all of the
bufferizations from npcomp (the exception being std.constant, which can
be upstreamed when std.global_memref lands:
https://llvm.discourse.group/t/rfc-global-variables-in-mlir/2076/16 )

Differential Revision: https://reviews.llvm.org/D90205