[Clang] Migrate llvm::make_unique to std::make_unique

[Clang] Migrate llvm::make_unique to std::make_unique

Now that we've moved to C++14, we no longer need the llvm::make_unique
implementation from STLExtras.h. This patch is a mechanical replacement
of (hopefully) all the llvm::make_unique instances across the monorepo.

Differential revision: https://reviews.llvm.org/D66259

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@368942 91177308-0d34-0410-b5e6-96231b3b80d8