Correctly detect if a symbol uses a reserved section index or not.

The logic was incorrect for variables, causing them to end up in the wrong
section if the section had an index >= 0xff00.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204771 91177308-0d34-0410-b5e6-96231b3b80d8