Clarify which type "Label" refers to (generic vs X86)

src/IceAssemblerARM32.h

Index: src/IceAssemblerARM32.h
diff --git a/src/IceAssemblerARM32.h b/src/IceAssemblerARM32.h
index 84f2da8467374f08942d85e3692b1b05a4fa5687..3ff597b3af3ad19c128cebfe80d7ea391d07fc47 100644
--- a/src/IceAssemblerARM32.h
+++ b/src/IceAssemblerARM32.h
@@ -61,7 +61,7 @@ public:
     llvm_unreachable("Not yet implemented.");
   }
 
-  Label *getOrCreateCfgNodeLabel(SizeT NodeNumber) override {
+  LabelBase *getCfgNodeLabel(SizeT NodeNumber) override {
     (void)NodeNumber;
     llvm_unreachable("Not yet implemented.");
   }