swift234

Only require a valid module name if we're actually producing a module.

- Massage error message to say 'module name "foo-bar" is not a valid
  identifier' instead of 'file name'.
- Don't require a valid module name for the main source file -- just
  fall back to "main" in that case. (This was always the intent; it
  just got lost in refactoring.)
- ...but don't just let an invalid module name slip by in this mode either.

<rdar://problem/11266169>

Swift SVN r7305