Fix a bug that undefined symbols are not reported if dead-stripping is enabled.

There was a bug that the linker does not report an error if symbols specified
by -u (or /include on Windows) are not resolved. This patch fixes it by adding
such symbols to the dead strip root.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@198041 91177308-0d34-0410-b5e6-96231b3b80d8