[llvm-objdump][NFC] Improve readability.

Summary:
Introduce a `struct SectionSymbol` instead of
`tuple<uint64_t, StringRef, uint8>`.

Reviewers: jhenderson, davide

Subscribers: rupprecht, llvm-commits

Differential Revision: https://reviews.llvm.org/D56858

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351529 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed