swift234

Cache map lookups

When we search for metadata we usually search for the same value over and
over again. Instead of searching the value in the binary tree we can save
the value that we found and use it again the next time we search. This
commit adds a cache value. The invariant is that the LastSearch variable
always points to a valid node and the access to the pointer itself is atomic.
This is a major performance win for the current use of getGenericMetadata.

Swift SVN r23434