code style AggregateFunctionMLMethod.cpp

SimpleAggregateFunction do not pass arena to the
add_function -> getAddressOfAddFunction(), hence next crash happens:
  (gdb) bt
  #0  DB::Arena::alloc (size=64, this=0x0) at ../dbms/src/Common/Arena.h:124
  #1  DB::SingleValueDataString::changeImpl (this=0x7f97424a27d8, value=..., arena=0x0) at ../dbms/src/AggregateFunctions/AggregateFunctionMinMaxAny.h:274
  #2  0x0000000005ea5319 in DB::AggregateFunctionNullUnary<true>::add (arena=<optimized out>, row_num=<optimized out>, columns=<optimized out>, place=<optimized out>, this=<optimized out>) at ../dbms/src/AggregateFunctions/AggregateFunctionNull.h:43
  #3  DB::IAggregateFunctionHelper<DB::AggregateFunctionNullUnary<true> >::addFree (that=<optimized out>, place=<optimized out>, columns=<optimized out>, row_num=<optimized out>, arena=<optimized out>) at ../dbms/src/AggregateFunctions/IAggregateFunction.h:131
  #4  0x000000000679772f in DB::AggregatingSortedBlockInputStream::addRow (this=this@entry=0x7f982de19c00, cursor=...) at ../dbms/src/Common/AlignedBuffer.h:31
  ClickHouse#5  0x0000000006797faa in DB::AggregatingSortedBlockInputStream::merge (this=this@entry=0x7f982de19c00, merged_columns=..., queue=...) at ../dbms/src/DataStreams/AggregatingSortedBlockInputStream.cpp:140
  ClickHouse#6  0x0000000006798979 in DB::AggregatingSortedBlockInputStream::readImpl (this=0x7f982de19c00) at ../dbms/src/DataStreams/AggregatingSortedBlockInputStream.cpp:78
  ClickHouse#7  0x000000000622db55 in DB::IBlockInputStream::read (this=0x7f982de19c00) at ../dbms/src/DataStreams/IBlockInputStream.cpp:56
  ClickHouse#8  0x0000000006613bee in DB::MergeTreeDataMergerMutator::mergePartsToTemporaryPart (this=this@entry=0x7f97ec65e1a0, future_part=..., merge_entry=..., time_of_merge=<optimized out>, disk_reservation=<optimized out>, deduplicate=<optimized out>) at /usr/include/c++/8/bits/shared_ptr_base.h:1018
  ClickHouse#9  0x000000000658f7a4 in DB::StorageReplicatedMergeTree::tryExecuteMerge (this=0x7f97ec65b810, entry=...) at /usr/include/c++/8/bits/unique_ptr.h:342
  ClickHouse#10 0x00000000065940ab in DB::StorageReplicatedMergeTree::executeLogEntry (this=0x7f97ec65b810, entry=...) at ../dbms/src/Storages/StorageReplicatedMergeTree.cpp:910
  <snip>

  (gdb) f 1
  (gdb) p MAX_SMALL_STRING_SIZE
  $1 = 48
  (gdb) p capacity
  $2 = 64
  (gdb) p value
  $3 = {data = 0x7f97242fcbd0 "HHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH", size = 61}

v2: avoid leaking of allocated by Arena memory on the intermediate step

Fixes: 8f8d2c0 ("Merge pull request ClickHouse#4629 from bgranvea/simple_aggregate_function")

Fixed style issues and build for clang-7;
64-bit buffer for BitReader and BitWriter;
Fixed overflow and writing more bytes on flushing;
Added maskLowBits() and tests for it.