[clang][driver] Fix multiarch output name with -Wl arg

Previously if you passed a `-Wl,-foo` _before_ the source filename, the
first `InputInfos`, which is used for the base input name would be an
`InputArg` kind, which would never have a base input name. Now we use
that by default, but pick the first `InputInfo` that is of kind
`Filename` to get the name from if there is one.

Differential Revision: https://reviews.llvm.org/D112767