[XCOFF][llvm-objdump] change the priority of symbols with

                      the same address by symbol types.

Summary: In XCOFF, each section comes with a default symbol
         with the same name as the section. It doesn't bind
         to code locations and it may cause incorrect display
         of symbol names under `llvm-objdump -d`.
         This patch changes the priority of symbols with the
         same address by symbol type.

Reviewed By: jhenderson, shchenz

Differential Revision: https://reviews.llvm.org/D117642