Object/COFF: Rename getXXX{Begin,End} -> xxx_{begin,end}.

It is mentioned in the LLVM coding standard that _begin() and _end() suffixes
should be used.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191569 91177308-0d34-0410-b5e6-96231b3b80d8