[clang][IFS] Ignoring -Xlinker/-Xclang arguments in InterfaceStubs pass for now.

Many of the inputs to clang driver toolchain tools can be things other
than files such as -Xclang and -Xlinker arguments. For now we don't take
such tool pass-through type arguments (although having an -Xifs would be nice to
have to replace things like -emit-merged-ifs). So because for the moment
we are not doing any sort of argument pass-through, I am going to have
InterfaceStubs Merger ignore any non-file type input arguments.

Differential Revision: https://reviews.llvm.org/D70838