Clang: For MSVC ABI do not use modes older than C++14

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

1 files changed, 1 insertions, 3 deletions

diff --git a/Tests/Module/WriteCompilerDetectionHeader/CMakeLists.txt b/Tests/Module/WriteCompilerDetectionHeader/CMakeLists.txt
index b30928d..cffef5a 100644
--- a/Tests/Module/WriteCompilerDetectionHeader/CMakeLists.txt
+++ b/Tests/Module/WriteCompilerDetectionHeader/CMakeLists.txt
@@ -133,9 +133,7 @@ endif()
 
 # for msvc the compiler version determines which c++11 features are available.
 if (CMAKE_CXX_COMPILER_ID STREQUAL "MSVC"
-    OR (CMAKE_CXX_COMPILER_ID STREQUAL "Clang"
-    AND "x${CMAKE_CXX_SIMULATE_ID}" STREQUAL "xMSVC"
-    AND "x${CMAKE_CXX_COMPILER_FRONTEND_VARIANT}" STREQUAL "xMSVC" ))
+    OR (CMAKE_CXX_COMPILER_ID STREQUAL "Clang" AND "x${CMAKE_CXX_SIMULATE_ID}" STREQUAL "xMSVC"))
   if(";${CMAKE_CXX_COMPILE_FEATURES};" MATCHES ";cxx_delegating_constructors;")
     list(APPEND true_defs EXPECTED_COMPILER_CXX_DELEGATING_CONSTRUCTORS)
     list(APPEND true_defs EXPECTED_COMPILER_CXX_VARIADIC_TEMPLATES)