[analyzer][CrossTU][NFC] Generalize to external definitions instead of external functions

Summary: This is just changing naming and documentation to be general about external definitions that can be imported for cross translation unit analysis. There is at least a plan to add VarDecls: D46421

Reviewers: NoQ, xazax.hun, martong, a.sidorin, george.karpenkov, serge-sans-paille

Reviewed By: xazax.hun, martong

Subscribers: mgorny, whisperity, baloghadamsoftware, szepet, rnkovacs, mikhail.ramalho, Szelethus, donat.nagy, dkrupp, cfe-commits

Differential Revision: https://reviews.llvm.org/D56441

llvm-svn: 350852