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.

llvm-svn: 54899
diff --git a/llvm/include/llvm-c/Core.h b/llvm/include/llvm-c/Core.h
index 7fa91cd..9ff1251 100644
--- a/llvm/include/llvm-c/Core.h
+++ b/llvm/include/llvm-c/Core.h
@@ -411,8 +411,8 @@
 unsigned LLVMGetIntrinsicID(LLVMValueRef Fn);
 unsigned LLVMGetFunctionCallConv(LLVMValueRef Fn);
 void LLVMSetFunctionCallConv(LLVMValueRef Fn, unsigned CC);
-const char *LLVMGetCollector(LLVMValueRef Fn);
-void LLVMSetCollector(LLVMValueRef Fn, const char *Coll);
+const char *LLVMGetGC(LLVMValueRef Fn);
+void LLVMSetGC(LLVMValueRef Fn, const char *Name);
 
 /* Operations on parameters */
 unsigned LLVMCountParams(LLVMValueRef Fn);