[libcxxabi] When catching an exception of type nullptr_t with a handler of

pointer-to-member type, produce a null value of the right type.

This fixes a bug where throwing an exception of type nullptr_t and catching it
as a pointer-to-member would not guarantee to produce a null value in the catch
handler. The fix is pretty simple: we statically allocate a constant null
pointer-to-data-member representation and a constant null
pointer-to-member-function representation, and produce the address of the
relevant value as the adjusted pointer for the exception.

llvm-svn: 276016