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
* Fix bug that <=> operator and in operator get wrong result
* Add some comment to get_result_for_null
* Add an new Binary Operator to replace is_safe_for_null for handleing '<=>' operator
* Add EQ_FOR_NULL to TExprOpcode
* Remove macro definition last backslash
* Add UserFunctionCache to cache UDF's library
This patch replace LibCache with UserFunctionCache. LibCache use HDFS
URL to identify a UDF's Library, and when BE process restart all of
downloaded library should be loaded another time. We use function id
corresponding to a library, and when process restart, all downloaded
libraries can be loaded without another downloading.
* update
