[GC] Style cleanup for RewriteStatepointForGC (1 of many) [NFC]

Starting to update variable naming and types to match LLVM style.  This will be an incremental process to minimize the chance of breakage as I work.  Step one, rename member variables to LLVM CamelCase and use llvm's ADT.  Much more to come.

llvm-svn: 230042