Change the behavior of the lvalue-to-rvalue conversion for varargs in...

Change the behavior of the lvalue-to-rvalue conversion for varargs in PotentiallyPotentiallyEvaluated contexts so that we model it in a sane way in most cases, and give up for the edge case which hopefully doesn't matter too much.

In preparation for correctly treating sizeof() as a PotentiallyPotentiallyEvaluated context.

llvm-svn: 148271