[Clang][Codegen] Truncate initializers of union bitfield members

If an initial value is given for a bitfield that does not fit in the
bitfield, the value should be truncated. Constant folding for
expressions did not account for this truncation in the case of union
member functions, despite a warning being emitted. In some contexts,
evaluation of expressions was not enabled unless C++11, ROPI or RWPI
was enabled.

Differential Revision: https://reviews.llvm.org/D93101