[clang-tidy][modernize-make-unique] Checks c++14 flag before using std::make_unique

Summary: For a c++11 code, the clang-tidy rule "modernize-make-unique" should return immediately, as std::make_unique is not supported.

Reviewers: hokein, aaron.ballman, ilya-biryukov, alexfh

Reviewed By: hokein, aaron.ballman, alexfh

Subscribers: Quuxplusone, xazax.hun, cfe-commits

Tags: #clang-tools-extra

Patch by Frederic Tingaud!

Differential Revision: https://reviews.llvm.org/D43766

llvm-svn: 328101