[PECOFF] Add files appear in .drectve to input graph

-- so that command line options to specify new input files, such as
/defaultlib:foo, is handled properly. Such options were ignored before
this patch.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@192342 91177308-0d34-0410-b5e6-96231b3b80d8