[ASTImporter] Handle redecl chain of FunctionTemplateDecls

Summary:
Redecl chains of function templates are not handled well currently. We
want to handle them similarly to functions, i.e. try to keep the
structure of the original AST as much as possible. The aim is to not
squash a prototype with a definition, rather we create both and put them
in a redecl chain.

Reviewers: a_sidorin, shafik, a.sidorin

Subscribers: rnkovacs, dkrupp, Szelethus, gamesh411, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58494

llvm-svn: 355593