Comment and minor code cleanup for GCStrategy (NFC)

Updating comments to reflect the current state of the world after my recent changes to ownership structure and generally better describe what a GCStrategy is and how it works.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224086 91177308-0d34-0410-b5e6-96231b3b80d8