[NFC] Make some ObjectFormatType switches covering

Summary:
This patch removes the `default` case from some switches on
`llvm::Triple::ObjectFormatType`, and cases for the missing enumerators
are then added.

For `UnknownObjectFormat`, the action (`llvm_unreachable`) for the
`default` case is kept.

For the other unhandled cases, `report_fatal_error` is used instead.

Reviewers: sfertile, jasonliu, daltenty

Reviewed By: sfertile

Subscribers: wuzish, aheejin, jsji, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D63767

llvm-svn: 365160