Remove unnecessary intrusive ref counting in favor of std::shared_ptr/make_shared

The intrusive nature of the reference counting is not required/used
here, so simplify the ownership model to make the code easier to
understand.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@291005 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed