llvm-cov: Explicitly namespace llvm::make_unique to keep MSVC happy

This is a followup to r208171, where a call to make_unique was
disambiguated for MSVC. Disambiguate two more calls, and remove the
comment about it since this is what we do everywhere.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208219 91177308-0d34-0410-b5e6-96231b3b80d8