[PECOFF] Do not scan the symbol table twice but instead cache aux symbols.

We scanned the symbol table twice; first to gather all regular symbols, and
second to process aux symbols. That's a bit inefficient and complicated. We
can instead cache aux symbols in the first pass, to eliminate the need of the
second pass.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@189525 91177308-0d34-0410-b5e6-96231b3b80d8