[lld] rename member variable name.

This renames variable name to reflect initial undefined symbols that are
defined by the linker -u option.

This doesnot change any functionality in lld, and updates code to reflect
Nick's comment.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@184682 91177308-0d34-0410-b5e6-96231b3b80d8