[PECOFF] Fix atom ordinals.

Atom ordinals are the indeces in a file. Currently the PECOFF reader assigns
ordinals for each section, so it's (incorrectly) assigning duplicate ordinals.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@195852 91177308-0d34-0410-b5e6-96231b3b80d8