Reapply "[IR] Move optional data in llvm::Function into a hungoff uselist"

Make personality functions, prefix data, and prologue data hungoff
operands of Function.

This is based on the email thread "[RFC] Clean up the way we store
optional Function data" on llvm-dev.

Thanks to sanjoyd, majnemer, rnk, loladiro, and dexonsmith for feedback!

Includes a fix to scrub value subclass data in dropAllReferences.

Differential Revision: http://reviews.llvm.org/D13829

llvm-svn: 256093