[PPC64] Fix RelType in checkInt and checkAlignment diagnsotics.

In the PPC64 target we map toc-relative relocations, dynamic thread pointer
relative relocations, and got relocations into a corresponding ADDR16 relocation
type for handling in relocateOne. This patch saves the orignal RelType before
mapping to an ADDR16 relocation so that any diagnostic messages will not
mistakenly use the mapped type.

Differential Revision: https://reviews.llvm.org/D56448

llvm-svn: 350827