Add driver option "-x <type>", which would treat subsequent input files as having the given type

This is a task for a FIXME left in swift/lib/Driver/Driver.cpp four years ago: "FIXME: add -x support (or equivalent)".

This is a reference to the Clang option -x, defined here: https://github.com/llvm-mirror/clang/blob/39691fddaf4f5199ecf9960ae3059943d59f659b/include/clang/Driver/Options.td#L2341-L2343

Currently, the Swift compiler determines the types of its inputs based on file name: files ending in a .sil extension are SIL files, .swift are Swift source files, and an empty or unrecognized extension is assumed to be an object file.

A -x option would allow users to invoke swiftc -x swift my-swift-source-file, such that my-swift-source-file would be processed as a Swift source file, even though it lacks a .swift extension.