Add comment to describe cleverness.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179806 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Basic/TargetInfo.cpp

@@ -373,6 +373,8 @@ bool TargetInfo::validateOutputConstraint(ConstraintInfo &Info) const {
     Name++;
   }
 
+  // If a constraint allows neither memory nor register operands it contains
+  // only modifiers. Reject it.
   return Info.allowsMemory() || Info.allowsRegister();
 }