[flang][runtime] Fix runtime CSHIFT of rank>1 array case of negative shift count

[klausler]

Summary

The calculation of the source index was incorrect when a CSHIFT shift
count value is negative, for the implementation of CSHIFT for arrays
with rank >= 2. (The vector CSHIFT is fine.)

flang/runtime/transformational.cpp

     }
     SubscriptValue &sourceDim{sourceAt[dim - 1]};
     sourceDim = dimLB + shiftCount % dimExtent;
-    if (shiftCount < 0) {
+    if (sourceDim < dimLB) {
       sourceDim += dimExtent;
     }
     for (resDim = 1; resDim <= dimExtent; ++resDim) {