Call normalize() in the common driver.

Previously only GNU driver calls InputGraph::normalize, but its
functionality is not and should not be limited to GNU ld. Other
driver should be able to use it.

Currently only linker scripts use the feature, so this change
won't change the existing behavior.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@208266 91177308-0d34-0410-b5e6-96231b3b80d8