swift234

[stdlib] Deal with various zero-division scenarios

Primarily, this means becoming resilient to Builtin.strideof(x) == 0.
Pretty much the only way to get pointers and arrays to make sense is to
treat zero-sized elements as having a stride of 1, so we do that in our
wrapper for Builtin.strideof.  Other points include precondition checks
for radixes in number formatting.

Fixes <rdar://problem/17097768>

Swift SVN r20242