[SPARK-12797] [SQL] Generated TungstenAggregate (without grouping keys)

As discussed in #10786, the generated TungstenAggregate does not support imperative functions.

For a query
```
sqlContext.range(10).filter("id > 1").groupBy().count()
```

The generated code will looks like:
```
/* 032 */     if (!initAgg0) {
/* 033 */       initAgg0 = true;
/* 034 */
/* 035 */       // initialize aggregation buffer
/* 037 */       long bufValue2 = 0L;
/* 038 */
/* 039 */
/* 040 */       // initialize Range
/* 041 */       if (!range_initRange5) {
/* 042 */         range_initRange5 = true;
       ...
/* 071 */       }
/* 072 */
/* 073 */       while (!range_overflow8 && range_number7 < range_partitionEnd6) {
/* 074 */         long range_value9 = range_number7;
/* 075 */         range_number7 += 1L;
/* 076 */         if (range_number7 < range_value9 ^ 1L < 0) {
/* 077 */           range_overflow8 = true;
/* 078 */         }
/* 079 */
/* 085 */         boolean primitive11 = false;
/* 086 */         primitive11 = range_value9 > 1L;
/* 087 */         if (!false && primitive11) {
/* 092 */           // do aggregate and update aggregation buffer
/* 099 */           long primitive17 = -1L;
/* 100 */           primitive17 = bufValue2 + 1L;
/* 101 */           bufValue2 = primitive17;
/* 105 */         }
/* 107 */       }
/* 109 */
/* 110 */       // output the result
/* 112 */       bufferHolder25.reset();
/* 114 */       rowWriter26.initialize(bufferHolder25, 1);
/* 118 */       rowWriter26.write(0, bufValue2);
/* 120 */       result24.pointTo(bufferHolder25.buffer, bufferHolder25.totalSize());
/* 121 */       currentRow = result24;
/* 122 */       return;
/* 124 */     }
/* 125 */
```

cc nongli

Author: Davies Liu <davies@databricks.com>

Closes #10840 from davies/gen_agg.