[clang-tidy] Fix one corner case in make-unique check.

Summary:
Previously, we tried to cover all "std::initializer_list" implicit conversion
cases in the code, but there are some corner cases that not covered (see
newly-added test in the patch).

Sipping all implicit AST nodes is a better way to filter out all these cases.

Reviewers: ilya-biryukov

Subscribers: klimek, xazax.hun, cfe-commits

Differential Revision: https://reviews.llvm.org/D44137

llvm-svn: 326799