[PCH] Remove the BackingIvarReferencedInAccessor field from DECL_OBJC_IVAR abbreviation record.

Patch by Yiding Jia!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211438 91177308-0d34-0410-b5e6-96231b3b80d8