Root causes were:
* Using incorrect conditions (assuming MSVC-like command line mode)
* Trying to compile the MSVC STL in C++11 mode, when parts of it require
C++14 or enabling MS extensions in clang.
* Missing flush in a testcase using stdout in a dll and a main part
with static crt
* 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.
