The configuration provider sends msvc intelliSenseMode for IAR compiler

Brief Issue Summary

When a compiler cmake-tools is not familiar with is used by CMake (e.g. an embedded compiler with a custom toolchain file), it ignores all other settings and defaults to the MSVC Intellisense mode on Windows. This is usually undesirable, since embedded code is not generally compatible with MSVC.

Expected:

There should be a way to manually override the intellisense settings on a project basis (possibly in cmake-kits.json?) so it can be used as a configuration provider even if the compiler being used by cmake is not supported.

C/C++ log file

In the log below it can be seen that the settings specified in c_cpp_properties.json are not respected and msvc intellisense mode is used when the IAR compiler is specified by a cmake toolchain.

-------- Diagnostics - 2/25/2020, 2:51:31 PM
Version: 0.26.3
Current Configuration:
{
    "name": "Win32",
    "includePath": [
        "${workspaceFolder}/**",
        "${workspaceFolder}/drivers/Include/**",
        "${workspaceFolder}/drivers/netx_drv/Include/**"
    ],
    "defines": [
        "_DEBUG",
        "UNICODE",
        "_UNICODE"
    ],
    "compilerPath": "C:\\Program Files (x86)\\GNU Tools Arm Embedded\\9 2019-q4-major\\bin\\arm-none-eabi-gcc.exe",
    "intelliSenseMode": "gcc-x64",
    "compilerArgs": [
        "-mthumb ",
        "-mcpu=cortex-m4"
    ],
    "configurationProvider": "vector-of-bool.cmake-tools",
    "browse": {
        "path": [
            "${workspaceFolder}/**",
            "${workspaceFolder}/drivers/Include/**",
            "${workspaceFolder}/drivers/netx_drv/Include/**"
        ],
        "limitSymbolsToIncludedHeaders": true
    }
}
Translation Unit Configurations:
[ C:\Users\pfox\ESeries\Firmware\firmware\CMSIS\Include\cmsis_iccarm.h ]:
    Process ID: 28372
    Memory Usage: 16 MB
    Compiler Path: C:/Program Files (x86)/Microsoft Visual Studio/2019/Professional/VC/Tools/MSVC/14.24.28314/bin/Hostx64/x64/cl.exe
    Includes:
        C:/Program Files (x86)/Microsoft Visual Studio/2019/Professional/VC/Tools/MSVC/14.24.28314/include/*
        C:/Program Files (x86)/Microsoft Visual Studio/2019/Professional/VC/Tools/MSVC/14.24.28314/atlmfc/include/*
        C:/Program Files (x86)/Windows Kits/10/Include/10.0.18362.0/um
        C:/Program Files (x86)/Windows Kits/10/Include/10.0.18362.0/ucrt
        C:/Program Files (x86)/Windows Kits/10/Include/10.0.18362.0/shared
        C:/Program Files (x86)/Windows Kits/10/Include/10.0.18362.0/winrt
        C:/Program Files (x86)/Windows Kits/10/Include/10.0.18362.0/cppwinrt
    Standard Version: ms_c++17
    IntelliSense Mode: msvc-x64
    Other Flags:
        --header_only_fallback
Total Memory Usage: 16 MB

Other Notes/Information

Clang may be a better fallback if the compiler can't be identified, since it's probably the most broadly compatible.

The issue names no source files or tests. Start by tracing how the configuration provider handles compiler detection and fallback IntelliSense settings, then determine where project-level overrides could be accepted. Done means an unsupported IAR compiler no longer forces msvc-x64 and the requested override is respected.