swift234

[driver] Use the input file name as the output executable name.

...when no -module-name or -o is provided.

This makes the single-file compile /very/ easy: compiling "foo.swift"
produces an output named "foo", rather than "a.out". For multi-file
modes, the module name is used if -o is missing; if no module name is
provided, it defaults to "main".

In a nutshell:
  explicit -o > explicit -module-name > single file's stem name > "main"

<rdar://problem/12593849>

Swift SVN r12841