Extract the (InputKind, std::string) pair used to describe inputs to

the front end into its own class, FrontendInputFile, to make it easier
to introduce new per-input data. No functionality change.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@148546 91177308-0d34-0410-b5e6-96231b3b80d8