Remove unused LLVM related codes of directory (step 3):be/src/exprs (#2910)
there are many LLVM related codes in code base, but these codes are not really used.
The higher version of GCC is not compatible with the LLVM 3.4.2 version currently used by Doris.
The PR delete all LLVM related code of directory: be/src/exprs
Add a new type: Object. Currently, it's mainly for complex aggregate metrics(HLL , Bitmap).
The Object type has the following constraints：
1 Object type could not as key column type
2 Object type doesn't support all indices (BloomFilter, short key, zone map, invert index)
3 Object type doesn't support filter and group by
In the implementation：
The Object type reuse the StringValue and StringVal, because in storage engine, the Object type is binary, it has a pointer and length.
