swift234

Remove SourceFile::getImports in favor of FileUnit::getImportedModules.

getImportedModules is the canonical way to get imports, whether private,
public, or both. This is especially true now that we have more flags
for SourceFile imports that really shouldn't be consumed by anyone
outside of SourceFile.

In this same vein, provide addImports instead of setImports, since imports
are always additive.

No visible functionality change.

Swift SVN r26634