Basic: [asmSymbolicName] follows the same rule as numbers in asm inputs

Input constraints like "0" and "[foo]" should be treated the same when
it comes to their corresponding output constraint.

This fixes PR21850.

llvm-svn: 225605

clang/lib/Basic/TargetInfo.cpp

@@ -583,6 +583,10 @@ bool TargetInfo::validateInputConstraint(ConstraintInfo *OutputConstraints,
       if (Info.hasTiedOperand() && Info.getTiedOperand() != Index)
         return false;
 
+      // A number must refer to an output only operand.
+      if (OutputConstraints[Index].isReadWrite())
+        return false;
+
       Info.setTiedOperand(Index, OutputConstraints[Index]);
       break;
     }