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
The underlying mechanism uses darwin's libcache library (if not building on darwin there is a default implementation).
This was originally introduced in SourceKit, but we are moving it to libswiftBasic so it can be utilized by libswiftIDE
for caching code-completion results.
Swift SVN r9610
