[CMake] Use correct include flag for MSVC

[phosek]

Summary

When building using the MSVC driver, we need to use /imsvc rather than
-isystem for system headers.

[paulkirth]

Is this change something we need to document or mention in release notes? I'm always blurry on what changes to build/config would require those.

Otherwise this is LGTM. You may want to see if @mstorsjo has any thoughts before landing, though.

[mstorsjo]

In D134864#3824071, @paulkirth wrote:

Is this change something we need to document or mention in release notes? I'm always blurry on what changes to build/config would require those.

Otherwise this is LGTM. You may want to see if @mstorsjo has any thoughts before landing, though.

Nothing to add from me here - thanks for checking!

compiler-rt/CMakeLists.txt

if(COMPILER_RT_USE_LLVM_UNWINDER)
   endif()
 endif()
 
 if (COMPILER_RT_CXX_LIBRARY STREQUAL "libcxx")
   # We are using the in-tree libc++ so avoid including the default one.
   append_list_if(COMPILER_RT_HAS_NOSTDINCXX_FLAG -nostdinc++ COMPILER_RT_COMMON_CFLAGS)
   append_list_if(COMPILER_RT_HAS_NOSTDLIBXX_FLAG -nostdlib++ COMPILER_RT_COMMON_LINK_FLAGS)
   # Use the in-tree libc++ through explicit include and library paths.
-  set(COMPILER_RT_CXX_CFLAGS "$<$<TARGET_EXISTS:cxx-headers>:-isystem$<JOIN:$<TARGET_PROPERTY:cxx-headers,INTERFACE_INCLUDE_DIRECTORIES>,$<SEMICOLON>-isystem>>")
+  set(COMPILER_RT_CXX_CFLAGS "$<$<TARGET_EXISTS:cxx-headers>:$<IF:$<BOOL:${MSVC}>,/imsvc,-isystem>$<JOIN:$<TARGET_PROPERTY:cxx-headers,INTERFACE_INCLUDE_DIRECTORIES>,$<SEMICOLON>$<IF:$<BOOL:${MSVC}>,/imsvc,-isystem>>>")
   if (COMPILER_RT_STATIC_CXX_LIBRARY)
     set(COMPILER_RT_CXX_LINK_LIBS "$<TARGET_LINKER_FILE:cxx_static>")
   else()
     set(COMPILER_RT_CXX_LINK_LIBS "$<TARGET_LINKER_FILE:$<IF:$<TARGET_EXISTS:cxx_shared>,cxx_shared,cxx_static>>")
   endif()
 elseif (COMPILER_RT_CXX_LIBRARY STREQUAL "none")
   # We aren't using any C++ standard library so avoid including the default one.
   append_list_if(COMPILER_RT_HAS_NOSTDINCXX_FLAG -nostdinc++ COMPILER_RT_COMMON_CFLAGS)

 elseif (SANITIZER_CXX_ABI_LIBNAME STREQUAL "libcxxabi")
   list(APPEND SANITIZER_CXX_ABI_LIBRARIES "c++abi")
 elseif (SANITIZER_CXX_ABI_LIBNAME STREQUAL "libstdc++")
   append_list_if(COMPILER_RT_HAS_LIBSTDCXX stdc++ SANITIZER_CXX_ABI_LIBRARIES)
 endif()
 
 if (SANITIZER_TEST_CXX_LIBNAME STREQUAL "libc++")
   if (SANITIZER_TEST_CXX_INTREE)
-    list(APPEND SANITIZER_TEST_CXX_CFLAGS "$<$<TARGET_EXISTS:cxx-headers>:-isystem$<JOIN:$<TARGET_PROPERTY:cxx-headers,INTERFACE_INCLUDE_DIRECTORIES>,$<SEMICOLON>-isystem>>")
+    list(APPEND SANITIZER_TEST_CXX_CFLAGS "$<$<TARGET_EXISTS:cxx-headers>:$<IF:$<BOOL:${MSVC}>,/imsvc,-isystem>$<JOIN:$<TARGET_PROPERTY:cxx-headers,INTERFACE_INCLUDE_DIRECTORIES>,$<SEMICOLON>$<IF:$<BOOL:${MSVC}>,/imsvc,-isystem>>>")
     if (SANITIZER_USE_STATIC_TEST_CXX)
       list(APPEND SANITIZER_TEST_CXX_LIBRARIES "$<TARGET_LINKER_FILE:cxx_static>")
     else()
       list(APPEND SANITIZER_TEST_CXX_LIBRARIES "$<TARGET_LINKER_FILE:$<IF:$<TARGET_EXISTS:cxx_shared>,cxx_shared,cxx_static>>")
     endif()
     # We are using the in tree libc++ so avoid including the default one.
     append_list_if(COMPILER_RT_HAS_NOSTDINCXX_FLAG -nostdinc++ COMPILER_RT_UNITTEST_CFLAGS)
     append_list_if(COMPILER_RT_HAS_NOSTDLIBXX_FLAG -nostdlib++ COMPILER_RT_UNITTEST_LINK_FLAGS)