swift234

[AST/IDE] Centralize the logic to determine if a symbols is a 'private' stdlib one and hide them more extensively.

-Hide vars that have a private type.
-Hide functions that have a parameter with private type or a parameter name with leading underscore.
-Minor change in StringUTF16.swift to avoid printing "func generate() -> IndexingGenerator<_StringCore>".

rdar://17027294

Swift SVN r18623