[mlir][bufferize] Improve analysis of external functions

[mlir][bufferize] Improve analysis of external functions

External functions have no body, so they cannot be analyzed. Assume conservatively that each tensor bbArg may be aliasing with each tensor result. Furthermore, assume that each function arg is read and written-to after bufferization. This default behavior can be controlled with `bufferization.access` (similar to `bufferization.memory_layout`) in test cases.

Also fix a bug in the dialect attribute verifier, which did not run for region argument attributes.

Differential Revision: https://reviews.llvm.org/D139517