Add IS_CXX option to sanitizer cmake module

[sfod]

Issue #, if available:

aws_check_sanitizer checks for the sanitizer flags using check_c_compiler_flag. However, for C++ projects C language could be not enabled, which leads to the following error:

CMake Error at /usr/share/cmake3/Modules/CheckCSourceCompiles.cmake:109 (try_compile):
  Unknown extension ".c" for file
    aws-iot-device-sdk-cpp-v2/samples/mqtt5/mqtt5_pubsub/cmake-build-tsan/CMakeFiles/CMakeTmp/src.c
  try_compile() works only for enabled languages.  Currently these are:
    CXX
  See project() command to enable other languages.

A possible solution could be to add enable_language(C) before calling check_c_compiler_flag. But I'm not sure that C++ and C compilers have the same set of the sanitizer options. And even if they do currently, this might change.

So, using check_cxx_compiler_flag for C++ project looks like a more correct approach.

Description of changes:

Add IS_CXX option to aws_check_sanitizer and aws_add_sanitizers CMake functions. This change does not break or change the existing usages of these functions.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.