[mlir][bufferize] Add filterFn option to BufferResultsToOutParams

This allows users to restrict the transformation to a
subset of the functions in a module.

For example, a user might want to apply the transformation to
a module's entry point, but not to the calls in the module
because those calls might refer to external C functions outside
of their control.

Reviewed By: springerm, nicolasvasilache
Differential Revision: https://reviews.llvm.org/D137264