Rename some GC classes so that their roll will hopefully be clearer.

In particular, Collector was confusing to implementors. Several
thought that this compile-time class was the place to implement
their runtime GC heap. Of course, it doesn't even exist at runtime.
Specifically, the renames are:

  Collector               -> GCStrategy
  CollectorMetadata       -> GCFunctionInfo
  CollectorModuleMetadata -> GCModuleInfo
  CollectorRegistry       -> GCRegistry
  Function::getCollector  -> getGC (setGC, hasGC, clearGC)

Several accessors and nested types have also been renamed to be
consistent. These changes should be obvious.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@54899 91177308-0d34-0410-b5e6-96231b3b80d8

lib/VMCore/Verifier.cpp

@@ -1308,8 +1308,8 @@ void Verifier::visitIntrinsicFunctionCall(Intrinsic::ID ID, CallInst &CI) {
         break;
       }
       
-      Assert1(CI.getParent()->getParent()->hasCollector(),
-              "Enclosing function does not specify a collector algorithm.",
+      Assert1(CI.getParent()->getParent()->hasGC(),
+              "Enclosing function does not use GC.",
               &CI);
     } break;
   case Intrinsic::init_trampoline: