Partially revert r189718 to add entry symbol to dead strip root.

Also added a test to verify that entry symbol is not stripped even if
dead stripping is enabled.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@189776 91177308-0d34-0410-b5e6-96231b3b80d8
Cosmetic changes

Differential Revision: http://llvm-reviews.chandlerc.com/D1577

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@189777 91177308-0d34-0410-b5e6-96231b3b80d8