[libc][NFC] Move tablegen indexer class into a util library of its own.

This class is currently used by two tools: HdrGen and PrototypeTestGen.
We will be adding more tools based on this class so it is convenient to
keep it in a util library of its own.

GitOrigin-RevId: 59f17a7d4f42fa330557e36d18f8b5f82a2c5e30
9 files changed