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

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