Frontend: Don't include stdin in the dependency list for an object file

GCC doesn't do this and it semes weird to include a file that we can't
open.

This fixes PR21362.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220726 91177308-0d34-0410-b5e6-96231b3b80d8