Since commit d44c0db0b2 (clang: setup correct configuration in gnu mode,
2019-02-20, v3.15.0-rc1~41^2~5) we support the GNU-like Clang that
targets the MSVC ABI. However, Clang cannot compile with the MSVC
standard library unless it runs in a mode aware of C++14 (since MSVC
itself does not even have a lower mode). When `CMAKE_CXX_STANDARD` is
set to 98 or 11, use C++14 anyway.
Since Clang's default mode is aware of C++14, another option is to not
add any flags for 98 or 11. However, if a future Clang version ever
defaults to a higher C++ standard, setting the standard to 98 or 11
should at least not use a mode higher than 14.
Also revert test updates from commit 4819ff9647 (Tests: fix failures
with gnu mode clang on windows, 2019-03-21, v3.15.0-rc1~41^2~3) that
were meant to work around the standard selection problem.
Fixes: #19496
d50b31be35