[PECOFF] Do not add the same library to the input graph more than once.

/defaultlib options can be specified implicitly via the .drectve section, and
it's pretty common that multiple object files add the same library, such as
user32.lib, to the input. We shouldn't add the same library multiple times.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@194129 91177308-0d34-0410-b5e6-96231b3b80d8