[ASTImporter] Add support for BuiltinTemplateDecl

Summary:
That decl kind is currently not implemented. BuiltinTemplateDecl is for decls that are hardcoded in the
ASTContext, so we can import them like we do other builtin decls by just taking the equivalent
decl from the target ASTContext.

Reviewers: martong, a.sidorin, shafik

Reviewed By: martong, shafik

Subscribers: rnkovacs, kristina, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69566