introduce clang CI and fix current link error

[Shikugawa]

After running make build with clang-10, we will see this link failure with messages as follows.

ERROR: /home/shikugawa/dev/authservice-test/src/main/BUILD:5:10: Linking of rule '//src/main:auth_server' failed (Exit 1): clang failed: error executing command /opt/llvm/bin/clang @bazel-out/k8-fastbuild/bin/src/main/auth_server-2.params

Use --sandbox_debug to see verbose messages from the sandbox clang failed: error executing command /opt/llvm/bin/clang @bazel-out/k8-fastbuild/bin/src/main/auth_server-2.params

Use --sandbox_debug to see verbose messages from the sandbox
bazel-out/k8-fastbuild/bin/external/boost/libboost_thread.a(thread.o):thread.cpp:function void boost::throw_exception<boost::bad_weak_ptr>(boost::bad_weak_ptr const&) [clone .isra.0]: error: relocation refers to global symbol "construction vtable for boost::exception_detail::clone_impl<boost::exception_detail::error_info_injector<boost::bad_weak_ptr> >-in-boost::wrapexcept<boost::bad_weak_ptr>", which is defined in a discarded section
  section group signature: "_ZTVN5boost10wrapexceptINS_12bad_weak_ptrEEE"
  prevailing definition is from bazel-out/k8-fastbuild/bin/src/common/http/_objs/http/http.pic.o
clang-10: error: linker command failed with exit code 1 (use -v to see invocation)
Target //src/main:auth_server failed to build
Use --verbose_failures to see the command lines of failed build steps.
INFO: Elapsed time: 88.731s, Critical Path: 46.83s
INFO: 2194 processes: 9 internal, 2185 linux-sandbox.
FAILED: Build did NOT complete successfully

Fix this problem and introduce clang CI as current gcc.

[incfly]

Resolved as the --config=clang becomes an option.