[PECOFF] Skip IMAGE_SYM_DEBUG sections correctly.

We don't use sections with IMAGE_SYM_DEBUG attribute so we basically
want to the symbols for them when reading symbol table. When we skip
them, we need to skip auxiliary symbols too. Otherwise weird error
would happen because aux symbols would be interpreted as regular ones.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@206931 91177308-0d34-0410-b5e6-96231b3b80d8