Require a GC strategy be specified for functions which use gc.statepoint

This was discussed a while back and I left it optional for migration.  Since it's been far more than the 'week or two' that was discussed, time to actually make this manditory.  



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233357 91177308-0d34-0410-b5e6-96231b3b80d8
10 files changed