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.
Run the `Utilities/Scripts/clang-format.bash` script to update
all our C++ code to a new style defined by `.clang-format`.
Use `clang-format` version 3.8.
* If you reached this commit for a line in `git blame`, re-run the blame
operation starting at the parent of this commit to see older history
for the content.
* See the parent commit for instructions to rebase a change across this
style transition commit.
