[Object] Rename getRelrRelocationType to getRelativeRelocationType

Summary:
The two utility functions were added in D47919 to support SHT_RELR.
However, these are just relative relocations types and are't
necessarily be named Relr.

Reviewers: phosek, dberris

Reviewed By: dberris

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D55691

llvm-svn: 349133