[mlir][bufferization] Enable BufferizationToMemRef to run on functions

Modify the `convert-bufferization-to-memref` pass to be applicable to
`builtin.module` and `FunctionOpInterface`. In many cases the lowering using
the library function is not needed because enough static information is
available to simplify the dealloc operations enough to use the more
light-weight lowerings. In those situations, it is better to allow the pass to
run on functions for additional parallelization and for easier use with the
transform dialect (e.g., when the transform ops are in the same
`builtin.module` as the function to be transformed).

Reviewed By: springerm

Differential Revision: https://reviews.llvm.org/D158969