swift234

Runtime: Use a per-cache bump allocator to instantiate metadata.

This has a couple benefits:

- Since metadata allocations are already guarded by a lock, the allocator doesn't require synchronization, and can be much much simpler and a little faster than malloc.
- By bypassing malloc, we also avoid tools like 'heap' prying into our metadata cache and misrepresenting cache entries keyed on classes as live objects, fixing rdar://problem/20562886.

In my unscientific local tests, this appeared to give a small across-the-board improvement to Onone performance in the perf test suite, though not far enough from noise for me to declare that definitively. Fixing the bug is the bigger point here.

Swift SVN r27856