Add comment to describe cleverness.

llvm-svn: 179806
diff --git a/clang/lib/Basic/TargetInfo.cpp b/clang/lib/Basic/TargetInfo.cpp
index 699f7d4..ff72f49 100644
--- a/clang/lib/Basic/TargetInfo.cpp
+++ b/clang/lib/Basic/TargetInfo.cpp
@@ -373,6 +373,8 @@
     Name++;
   }
 
+  // If a constraint allows neither memory nor register operands it contains
+  // only modifiers. Reject it.
   return Info.allowsMemory() || Info.allowsRegister();
 }