swift234

runtime: Use hashtables for metadata caches (again).

Using a linked list for metadata caches is pretty lame. Pull in llvm::DenseMap and hash_combine_range and use them to index instantiated metadata.

The previous attempt at this failed because tuple type metadata was laid out in a way that smashed the metadata cache key. Cache keys used to be laid out like this:

CacheEntry struct
-----------------
cache key
-----------------
variable-sized payload

And TupleTypeMetadata tried to tail-emplace its element array immediately after the main CacheEntry, forgetting the cache key was there. When we actually try to use that cache key to implement a hash table, bad things happen. Rearrange cache entries into the less error-prone layout:

cache key
-----------------
CacheEntry struct
-----------------
variable-sized payload

This also nicely avoids the need for a dynamic offset from the CacheEntry struct to its payload. A tail-allocated payload is likely to be more hot than the cache key, which is only needed at instantiation and lookup time.

Swift SVN r13205