swift234

Clarify SourceFile vs. TranslationUnit interfaces for SILGen and IRGen.

Once we have multiple SourceFiles in a TranslationUnit, it no longer makes
sense to say "only SILGen decls starting from element N" without specifying
which source file you mean.

Also, clarify ownership by having performSILGeneration return a unique_ptr
instead of just a bare pointer.

Swift SVN r9112