* Disable the system include unused variable test in ExportImport when
clang is in MSVC compatible mode.
* Disable CxxDialect testcase when clang is in MSVC compatible mode, as
it doesn't support `typeof`.
* Teach Module.WriteCompilerDetectionHeader to treat clang-cl as MSVC.
* Disable the SystemIncludeDirectories testcase within
IncludeDirectories when clang is in MSVC compatible mode.
* Disable the CMakeOnly.CheckCXXCompilerFlag testcase when clang is in
MSVC compatible mode.
* Treat clang-cl as MSVC in LinkOptions.cmake in the try_run and
try_compile testcases.
Since commit v3.3.0-rc1~397^2 (Check*CompilerFlag: Refactor method used
to pass flags, 2015-02-24) these check modules pass the flags to the
compiler front-end during linking as well as during compilation. This
breaks checks for flags like '-x c++' that are meant only for the
compilation step. Revert the change and add a test covering a
compiler-only flag.
