[benchmark] Add versions of prims that in a non-efficient way uses various forms of non-strong references.

Specifically, I add some benchmarks for weak, unowned, unsafe (unowned), and
unmanaged. The reason for the split in between unsafe (unowned) and unmanaged is
that one is testing the raw compiler features and the other is validating stdlib
performance.

Author: gottesmm

benchmark/CMakeLists.txt

@@ -131,6 +131,7 @@ set(SWIFT_BENCH_MODULES
     single-source/Prefix
     single-source/PrefixWhile
     single-source/Prims
+    single-source/PrimsNonStrongRef
     single-source/ProtocolDispatch
     single-source/ProtocolDispatch2
     single-source/Queue