Update the comments on default subtargets based on feedback.

llvm-svn: 309041
diff --git a/llvm/lib/Target/ARM/ARMTargetMachine.h b/llvm/lib/Target/ARM/ARMTargetMachine.h
index 22ce949..bc4c751b 100644
--- a/llvm/lib/Target/ARM/ARMTargetMachine.h
+++ b/llvm/lib/Target/ARM/ARMTargetMachine.h
@@ -47,8 +47,9 @@
   ~ARMBaseTargetMachine() override;
 
   const ARMSubtarget *getSubtargetImpl(const Function &F) const override;
-  // The no argument getSubtargetImpl, while it exists on some targets, is
-  // deprecated and should not be used.
+  // DO NOT IMPLEMENT: There is no such thing as a valid default subtarget,
+  // subtargets are per-function entities based on the target-specific
+  // attributes of each function.
   const ARMSubtarget *getSubtargetImpl() const = delete;
   bool isLittleEndian() const { return isLittle; }