[lldb][NFC] Simplify part of Options::GenerateOptionUsage

Use llvm::enumerate, remove an unused arg name stream and
replace repeated uses of indexing to get the option def.

We could use map instead of multimap but I'm not 100% that
would be NFC. All short options should be unique in theory.

Depends on D123500

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D123501