Our libcache implementation of swift::sys::Cache was broken for
ref-counted values (which are used by e.g. the SourceKit ASTManager).
It would always `retain(value)` in `set(key, value)`, but under the hood
libcache shares values, so we would only get one `release(value)` if the
same value was used across multiple keys, or if the same value *and* key
were set multiple times.
This was causing us to never release ASTs cached by SourceKit even when
the underlying libcache purged itself under memory pressure.
rdar://problem/21619189
