Teach UninitializedValuesV2 to implicitly reason about C++

references by monitoring whether an access to
a variable is solely to compute it's lvalue or
to do an lvalue-to-rvalue conversion (i.e., a load).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@123777 91177308-0d34-0410-b5e6-96231b3b80d8