Adds an option to allow modification of public functions to the drop
equivalent buffer results API and the respective pass.
A new standalone test for the pass is also added.
https://github.com/llvm/llvm-project/pull/163001 modified the default
behavior by disallowing rewriting public function. This PR preserves the
new behavior and only adds an opt-in flag to reenable changing public
function.
The extra flag aligns dropping equivalent function results with another
bufferization API `promoteBufferResultsToOutParams` which also allows
opt-in public function rewriting.
9a767dce95