Reject asm output constraints that consist of modifiers only.

Fixes PR15759.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179756 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Basic/TargetInfo.cpp b/lib/Basic/TargetInfo.cpp
index 70ea235..699f7d4 100644
--- a/lib/Basic/TargetInfo.cpp
+++ b/lib/Basic/TargetInfo.cpp
@@ -373,7 +373,7 @@
     Name++;
   }
 
-  return true;
+  return Info.allowsMemory() || Info.allowsRegister();
 }
 
 bool TargetInfo::resolveSymbolicName(const char *&Name,