Switch MetadataCache to use a global slab allocator.

[rjmccall]

This seems to more than fix a performance regression that we
detected on a metadata-allocation microbenchmark.

A few months ago, I improved the metadata cache representation
and changed the metadata allocation scheme to primarily use malloc.
Previously, we'd been using malloc in the concurrent tree data
structure but a per-cache slab allocator for the metadata itself.
At the time, I was concerned about the overhead of per-cache
allocators, since many metadata patterns see only a small number
of instantiations. That's still an important factor, so in the
new scheme we're using a global allocator; but instead of using
malloc for individual allocations, we're using a slab allocator,
which should have better peak, single-thread performance, at the
cost of not easily supporting deallocation. Deallocation is
only used for metadata when there's contention on the cache, and
specifically only when there's contention for the same key, so
leaking a little isn't the worst thing in the world.

The initial slab is a 64K globally-allocated buffer.
Successive slabs are 16K and allocated with malloc.

rdar://28189496

This is a resubmission of #7109, now that I believe the Linux build-system problems have been fixed in #7147.

[rjmccall]

@swift-ci Please test.