HandleGetInputList API is not implementable without memory-safety errors

[bzbarsky-apple]

Problem

This API asks the callee to return a list of structs. Those structs contains CharSpan instances, but there is nowhere to store the data those CharSpans point to, and no way to know when it's safe to free it. This means the API cannot be implemented without either leaks or use-after-free, or both.

Proposed Solution

Fix the API to be implementable without leaks or security bugs.

@chrisdecenzo @lazarkov @xylophone21

[bzbarsky-apple]

This API was added in #13259

[lazarkov]

This one should be fixed now. @bzbarsky-apple
If not please re-open.