CollectorMetadata and Collector are rejiggered to get along with

CollectorMetadata and Collector are rejiggered to get along with
per-function collector model. Collector is now the factory for
CollectorMetadata, so the latter may be subclassed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@44827 91177308-0d34-0410-b5e6-96231b3b80d8