Revert "[compiler-rt] Migrate llvm::make_unique to std::make_unique"

The X-ray unit tests in compiler-rt are overriding the C++ version by
explicitly passing -std=c++11 in the compiler invocation. This poses a
problem as these tests are including LLVM headers that can now use C++14
features. I'm temporarily reverting this as I investigate the correct
solution.

llvm-svn: 368952