swift234

[stdlib] Make algorithms safe with illegal predicates

Even if the user supplies an ordering predicate that isn't a strict-weak
ordering, algorithms should not index beyond their bounds.  Otherwise, a
use of withUnsafeMutableStorage for optimization purposes could easily
do an unsafe memory access.

This commit comments and tests our algorithms that require strict weak
orderings, and fixes safety problems in partition().  Most benchmarks
are unaffected, but the rewrite of partition produces a 27% speedup in
the Phonebook benchmark at -O3 and a 22% speedup at -Ofast.

Also, at -Ofast, QuickSort lost 6% and RC4 gained 6%.  These benchmarks
were not noticeably affected at -O3

====================`PrecommitBench_O3`====================
````benchmark`,`baserun0`,`baserun1`,`baserun2`,``optrun0`,``optrun1`,``optrun2`,``delta`,`speedup`
````Phonebook`,``1608.00`,``1676.00`,``1651.00`,``1265.00`,``1278.00`,``1281.00`,`343.00`,```27.1%`
````QuickSort`,```430.00`,```448.00`,```429.00`,```428.00`,```431.00`,```428.00`,```1.00`,````0.2%`
``````````RC4`,```925.00`,```924.00`,```922.00`,```916.00`,```919.00`,```917.00`,```6.00`,````0.7%`
====================`PrecommitBench_Ofast`====================
````benchmark`,`baserun0`,`baserun1`,`baserun2`,``optrun0`,``optrun1`,``optrun2`,``delta`,`speedup`
````Phonebook`,``1521.00`,``1546.00`,``1591.00`,``1252.00`,``1255.00`,``1256.00`,`269.00`,```21.5%`
````QuickSort`,```478.00`,```477.00`,```476.00`,```506.00`,```510.00`,```513.00`,``30.00`,```-5.9%`
``````````RC4`,``1033.00`,``1874.00`,``1030.00`,```974.00`,```982.00`,```975.00`,``56.00`,````5.7%`

Swift SVN r20202