clang-cl: Use .obj as extension for unnamed object files (PR17095)

We already use .obj as extension when the user provides a stem file
name (via /Fo), but were failing in the most basic case when the file
name is based on the input file.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190071 91177308-0d34-0410-b5e6-96231b3b80d8