Summary:
When building with clang 9, warning is reported for InternalDBStatsType type names shadowed the one for statistics. Rename them.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5779

Test Plan: Build with clang 9 and see it passes.

Differential Revision: D17239378

fbshipit-source-id: af28fb42066c738cd1b841f9fe21ab4671dafd18