swift234

[driver] Treat "main.swift" as the main source file if given multiple inputs.

New rules for the driver (first match):
1. -repl: no input files allowed
2. -parse-sil: one input file allowed
3. -parse-as-library: any number of input files, all treated as Library
4. one input, extension is .sil: treated as SIL
5. one input: treated as Main
6. many inputs: treated as Library by default; "main.swift" is treated as Main

If we want more control here we can also add a -main-file option to explicitly
call out the main source file, but this at least unblocks building an entire
app target (like ListMaker) with a single Swift invocation.

Swift SVN r9890