Remove unused LLVM related codes of directory (step 5):be/src/codegen (#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/codegen
In our storage engine's code, we cast StringValue to Slice. Because
their memory layout is different, it may cause BE process crash.
We make their memory layout same in this patch to resolve this problem
temporary. We should improve it some day.
