[LCG] Stop playing fast and loose with reference members and assignment.
It doesn't work. I'm still cleaning up all the places where I blindly
followed this pattern. There are more to come in this code too.

As a benefit, this lets the default copy and move operations Just Work.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206375 91177308-0d34-0410-b5e6-96231b3b80d8