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

6 files changed, 8 insertions, 30 deletions

diff --git a/Tests/AliasTarget/CMakeLists.txt b/Tests/AliasTarget/CMakeLists.txt
index 6271988..fc70135 100644
--- a/Tests/AliasTarget/CMakeLists.txt
+++ b/Tests/AliasTarget/CMakeLists.txt
@@ -7,7 +7,7 @@ set(CMAKE_CXX_STANDARD 98)
 # Clang/C2 in C++98 mode cannot properly handle some of MSVC headers
 if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang" AND
     CMAKE_CXX_SIMULATE_ID STREQUAL "MSVC")
-  set(CMAKE_CXX_STANDARD 14)
+  set(CMAKE_CXX_STANDARD 11)
 endif()
 
 add_library(foo SHARED empty.cpp)
diff --git a/Tests/Complex/CMakeLists.txt b/Tests/Complex/CMakeLists.txt
index fef83f6..2e41754 100644
--- a/Tests/Complex/CMakeLists.txt
+++ b/Tests/Complex/CMakeLists.txt
@@ -446,11 +446,6 @@ if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang" AND
   set(CMAKE_CXX_STANDARD 11)
 endif()
 
-if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang" AND
-    CMAKE_CXX_COMPILER_FRONTEND_VARIANT STREQUAL "GNU")
-  set(CMAKE_CXX_STANDARD 14)
-endif()
-
 #
 # Create the libs and the main exe
 #
diff --git a/Tests/ComplexOneConfig/CMakeLists.txt b/Tests/ComplexOneConfig/CMakeLists.txt
index 77baa4c..628cd4e 100644
--- a/Tests/ComplexOneConfig/CMakeLists.txt
+++ b/Tests/ComplexOneConfig/CMakeLists.txt
@@ -403,11 +403,6 @@ if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang" AND
   set(CMAKE_CXX_STANDARD 11)
 endif()
 
-if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang" AND
-    CMAKE_CXX_COMPILER_FRONTEND_VARIANT STREQUAL "GNU")
-  set(CMAKE_CXX_STANDARD 14)
-endif()
-
 #
 # Create the libs and the main exe
 #
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)
diff --git a/Tests/Plugin/CMakeLists.txt b/Tests/Plugin/CMakeLists.txt
index c4540db..8e8fa07 100644
--- a/Tests/Plugin/CMakeLists.txt
+++ b/Tests/Plugin/CMakeLists.txt
@@ -5,17 +5,6 @@ project(Plugin)
 # We need proper C++98 support from the compiler
 set(CMAKE_CXX_STANDARD 98)
 
-# Clang/C2 in C++98 mode cannot properly handle some of MSVC headers
-if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang" AND
-    CMAKE_CXX_SIMULATE_ID STREQUAL "MSVC")
-  set(CMAKE_CXX_STANDARD 11)
-endif()
-
-if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang" AND
-    CMAKE_CXX_COMPILER_FRONTEND_VARIANT STREQUAL "GNU")
-  set(CMAKE_CXX_STANDARD 14)
-endif()
-
 # Test per-target output directory properties.
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${Plugin_BINARY_DIR}/bin)
 set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${Plugin_BINARY_DIR}/lib/plugin)
@@ -40,6 +29,12 @@ include_directories(
   ${Plugin_SOURCE_DIR}/include
   )
 
+# Clang/C2 in C++98 mode cannot properly handle some of MSVC headers
+if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang" AND
+    CMAKE_CXX_SIMULATE_ID STREQUAL "MSVC")
+  set(CMAKE_CXX_STANDARD 11)
+endif()
+
 # Create an executable that exports an API for use by plugins.
 add_executable(example_exe src/example_exe.cxx)
 set_target_properties(example_exe PROPERTIES
diff --git a/Tests/RunCMake/GenerateExportHeader/GEH.cmake b/Tests/RunCMake/GenerateExportHeader/GEH.cmake
index b3f1c7f..ae9a84c 100644
--- a/Tests/RunCMake/GenerateExportHeader/GEH.cmake
+++ b/Tests/RunCMake/GenerateExportHeader/GEH.cmake
@@ -51,11 +51,6 @@ if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang" AND
   set(CMAKE_CXX_STANDARD 11)
 endif()
 
-if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang" AND
-    CMAKE_CXX_COMPILER_FRONTEND_VARIANT STREQUAL "GNU")
-  set(CMAKE_CXX_STANDARD 14)
-endif()
-
 add_subdirectory(lib_shared_and_static)
 
 if(CMAKE_SYSTEM_NAME MATCHES "AIX" AND CMAKE_CXX_COMPILER_ID STREQUAL "GNU"