[lld][LayoutPass] Split buildFollowOnTable for readability.

Summary:
Split buildFollowOnTable to small functions to improve
code readability and remove code duplication. No change
in functionality.

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D790

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@181749 91177308-0d34-0410-b5e6-96231b3b80d8