[clang-tidy] Fix make-unique tests on C++2a.

Summary:
These test cases are illgal in C++2a ("new Foo{}" needs to see the
default constructor), so move them to the C++14-only tests.

Reviewers: gribozavr

Subscribers: xazax.hun, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62845

llvm-svn: 362679