[mlir][bufferize] Better user control of layout maps

This changes replaces the `fully-dynamic-layout-maps` options (which was badly named) with two new options:

* `unknown-type-conversion` controls the layout maps on buffer types for which no layout map can be inferred.
* `function-boundary-type-conversion` controls the layout maps on buffer types inside of function signatures.

Differential Revision: https://reviews.llvm.org/D125615