Expression statements undergo lvalue-to-rvalue conversion in C,

Expression statements undergo lvalue-to-rvalue conversion in C,
but not in C++, so don't emit aggregate loads of volatile references
in null context in C++.  Happens to have been caught by an assertion.
We do not get the scalar case right.  Volatiles are really broken.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@112019 91177308-0d34-0410-b5e6-96231b3b80d8