The only time we would consider allowing this is inside a call to
std::allocator<T>::deallocate, whose contract does not permit deletion
of null pointers.
during the same evaluation.
It looks like the only case for which this matters is determining
whether mutable subobjects of a heap allocation can be modified during
constant evaluation.
For now, we restrict this support to use from within the standard
library implementation, since we're required to make parts of the
standard library that use placement new work, but not permitted to
make uses of placement new from user code work.
llvm-svn: 373547
