Plugins/ObjectFile/PECOFF: Use enums from LLVM, and don't use Mach-O definitions.

Since I renamed most of the LLVM Mach-O enums in r189314, I had to go fix
LLDB to use the new names. While I was here, I decided that a COFF
plugin really shouldn't be using Mach-O enums.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@189316 91177308-0d34-0410-b5e6-96231b3b80d8