[ci][SWIG] fixed SWIG -Wstrict-aliasing warnings

[StrikerRUS]

Fixed #1556.

Clang is OK too:

Log (clickable)

-- The C compiler identification is Clang 7.1.0
-- The CXX compiler identification is Clang 7.1.0
-- Check for working C compiler: /usr/bin/clang
-- Check for working C compiler: /usr/bin/clang -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/clang++
-- Check for working CXX compiler: /usr/bin/clang++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Found SWIG: /usr/local/bin/swig (found version "3.0.12") 
-- Found Java: /usr/lib/jvm/java-8-openjdk-amd64/bin/java (found version "1.8.0_171") 
-- Found JNI: /usr/lib/jvm/java-8-openjdk-amd64/jre/lib/amd64/libjawt.so  
-- Found OpenMP_C: -fopenmp=libomp (found version "3.1") 
-- Found OpenMP_CXX: -fopenmp=libomp (found version "3.1") 
-- Found OpenMP: TRUE (found version "3.1")  
CMake Deprecation Warning at /usr/local/share/cmake-3.14/Modules/UseSWIG.cmake:559 (message):
  SWIG_ADD_MODULE is deprecated.  Use SWIG_ADD_LIBRARY instead.
Call Stack (most recent call first):
  CMakeLists.txt:186 (swig_add_module)


CMake Warning (dev) at /usr/local/share/cmake-3.14/Modules/UseSWIG.cmake:600 (message):
  Policy CMP0078 is not set: UseSWIG generates standard target names.  Run
  "cmake --help-policy CMP0078" for policy details.  Use the cmake_policy
  command to set the policy and suppress this warning.

Call Stack (most recent call first):
  /usr/local/share/cmake-3.14/Modules/UseSWIG.cmake:560 (swig_add_library)
  CMakeLists.txt:186 (swig_add_module)
This warning is for project developers.  Use -Wno-dev to suppress it.

CMake Warning (dev) at /usr/local/share/cmake-3.14/Modules/UseSWIG.cmake:460 (message):
  Policy CMP0086 is not set: UseSWIG honors SWIG_MODULE_NAME via -module
  flag.  Run "cmake --help-policy CMP0086" for policy details.  Use the
  cmake_policy command to set the policy and suppress this warning.

Call Stack (most recent call first):
  /usr/local/share/cmake-3.14/Modules/UseSWIG.cmake:695 (SWIG_ADD_SOURCE_TO_MODULE)
  /usr/local/share/cmake-3.14/Modules/UseSWIG.cmake:560 (swig_add_library)
  CMakeLists.txt:186 (swig_add_module)
This warning is for project developers.  Use -Wno-dev to suppress it.

-- Configuring done
-- Generating done
-- Build files have been written to: /__w/1/s/build
Scanning dependencies of target _lightgbm_swig_swig_compilation
[  1%] Swig compile swig/lightgbmlib.i for java
Scanning dependencies of target lightgbm
Scanning dependencies of target _lightgbm
[  3%] Building CXX object CMakeFiles/_lightgbm.dir/src/c_api.cpp.o
[  4%] Building CXX object CMakeFiles/lightgbm.dir/src/main.cpp.o
[  6%] Building CXX object CMakeFiles/lightgbm.dir/src/application/application.cpp.o
[  6%] Built target _lightgbm_swig_swig_compilation
[  7%] Building CXX object CMakeFiles/_lightgbm.dir/src/lightgbm_R.cpp.o
[  9%] Building CXX object CMakeFiles/lightgbm.dir/src/boosting/boosting.cpp.o
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[ 72%] Building CXX object CMakeFiles/_lightgbm.dir/src/objective/objective_function.cpp.o
[ 74%] Building CXX object CMakeFiles/_lightgbm.dir/src/treelearner/data_parallel_tree_learner.cpp.o
[ 75%] Building CXX object CMakeFiles/lightgbm.dir/src/network/network.cpp.o
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[ 89%] Linking CXX shared library ../lib_lightgbm.so
[ 89%] Built target _lightgbm
Scanning dependencies of target _lightgbm_swig
[ 90%] Building CXX object CMakeFiles/_lightgbm_swig.dir/java/lightgbmlibJAVA_wrap.cxx.o
[ 92%] Building CXX object CMakeFiles/lightgbm.dir/src/treelearner/gpu_tree_learner.cpp.o
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[ 96%] Building CXX object CMakeFiles/lightgbm.dir/src/treelearner/voting_parallel_tree_learner.cpp.o
[ 98%] Linking CXX shared module ../lib_lightgbm_swig.so
[ 98%] Built target _lightgbm_swig
[100%] Linking CXX executable ../lightgbm
[100%] Built target lightgbm

@imatiach-msft Please review.

[imatiach-msft]

LGTM!

[imatiach-msft]

@StrikerRUS signed off, thanks for fixing this!