[analyzer][CrossTU] Extend CTU to VarDecls with initializer

Summary:
The existing CTU mechanism imports FunctionDecls where the definition is available in another TU. This patch extends that to VarDecls, to bind more constants.

• Add VarDecl importing functionality to CrossTranslationUnitContext
• Import Decls while traversing them in AnalysisConsumer
• Add VarDecls to CTU external mappings generator
• Name changes from "external function map" to "external definition map"

Reviewers: NoQ, dcoughlin, xazax.hun, george.karpenkov, martong

Reviewed By: xazax.hun

Subscribers: Charusso, baloghadamsoftware, mikhail.ramalho, Szelethus, donat.nagy, dkrupp, george.karpenkov, mgorny, whisperity, szepet, rnkovacs, a.sidorin, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D46421