[PATCH] [TargetInfo] Move LinkerInputFile to TargetInfo and rename more specific name

[kledzik at apple.com]

  It is fine to have the vector of undefined symbol names be common down in TargetInfo, but the use of it to create atoms should be done in the TargetInfo subclasses where target specific atoms are available.  You could virtualize it where TargetInfo calls an abstract method on itself passing each undefined symbol name.  The TargetInfo subclasses then get a chance to instantiate target specific atoms.

  I would like to see TargetInfo::_undefinedSymbols be named _initialUndefinedSymbols.  The set of undefined symbols changes throughout the link.  Adding "initial" helps clarify what it does.

http://llvm-reviews.chandlerc.com/D1016