[ThinLTO] Promotion handling cleanup (NFC)

Summary:
Clean up the code that does GV promotion in the ThinLTO backends.

Specifically, we don't need to check whether we are importing since that
is already checked and handled correctly in shouldPromoteLocalToGlobal.
Simply call shouldPromoteLocalToGlobal, and if it returns true we are
guaranteed that we are promoting, whether or not we are importing (or in
the exporting module). This also makes the handling in getName()
consistent with that in getLinkage(), which checks the DoPromote parameter
regardless of whether we are importing or exporting.

Reviewers: steven_wu, pcc, evgeny777

Subscribers: mehdi_amini, inglorion, hiraditya, dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70327