[ASTImporter] Fix import of VarDecl init

Summary:
The init expression of a VarDecl is overwritten in the "To" context if we
import a VarDecl without an init expression (and with a definition).  Please
refer to the added tests, especially InitAndDefinitionAreInDifferentTUs.  This
patch fixes the malfunction by importing the whole Decl chain similarly as we
did that in case of FunctionDecls.  We handle the init expression similarly to
a  definition, alas only one init expression will be in the merged ast.

Reviewers: a_sidorin, xazax.hun, r.stahl, a.sidorin

Subscribers: rnkovacs, dkrupp, cfe-commits

Differential Revision: https://reviews.llvm.org/D51597

llvm-svn: 342384