C23 features aren't supported

Environment

• OS and Version: Ubuntu 22.04
• VS Code Version: 1.76.2
• C/C++ Extension Version: 1.15.0
• If using SSH remote, specify OS of remote machine:

Bug Summary and Steps to Reproduce

Bug Summary:

There was an issue (#10615) that allows you to select c23/gnu23 as your compiler version, but the actual new language features are not implemented yet. Obviously this stuff is bleeding edge, but I struggled trying to get it to work and I wanted to submit an issue.

I would be happy to work on adding these features to the C/C++ language server, but I would need to be pointed in the right direction.

Steps to reproduce:

1. Set your C standard version to c23 or gnu23
2. Open the following code with IntelliSense active

enum TestEnum : short
{
    TEST_ENUM_VALUE,
};

void doSomething() {
    int * test1 = nullptr;
    bool test2 = true; // without #include <stdbool.h>
    enum TestEnum test3 = TEST_ENUM_VALUE;
}

3. Verify that these all generate "Problems"
   

4. Compile with -std=gnu2x or equivalent, on a very new compiler (I used clang 17) and verify that there are no errors

Expected behavior:
Generate the same errors that clang 17 would, or equivalent

Configuration and Logs

{
    "configurations": [
        {
            "name": "Linux",
            "compilerPath": "/usr/bin/clang-17",
            "cStandard": "c23",
            "compileCommands": "${workspaceFolder}/build/compile_commands.json"
        }
    ],
    "version": 4
}

-------- Diagnostics - 3/17/2023, 1:56:58 AM
Version: 1.15.0
Current Configuration:
{
    "name": "Linux",
    "compilerPath": "/usr/bin/clang-17",
    "cStandard": "c23",
    "compileCommands": "WORKSPACE/build/compile_commands.json",
    "compilerPathIsExplicit": true,
    "cStandardIsExplicit": true,
    "cppStandardIsExplicit": false,
    "intelliSenseModeIsExplicit": false,
    "rawCompilerPath": "/usr/bin/clang-17",
    "mergeConfigurations": false,
    "browse": {
        "limitSymbolsToIncludedHeaders": true
    }
}
Translation Unit Mappings:
[ WORKSPACE/src/test.c ]:
    WORKSPACE/src/test.c
Translation Unit Configurations:
[ WORKSPACE/src/test.c ]:
    Process ID: 3852781
    Memory Usage: 544 MB
    Compiler Path: /usr/bin/clang-17
    Includes:
        /usr/lib/llvm-17/lib/clang/17/include
        /usr/local/include
        /usr/include/x86_64-linux-gnu
        /usr/include
    Standard Version: c23
    IntelliSense Mode: linux-clang-x64
    Other Flags:
        --clang
        --clang_version=170000
    compile_commands.json entry:
        directory: WORKSPACE/build
        file: WORKSPACE/src/test.c
        command: /usr/bin/clang   -g -std=gnu2x -o WORKSPACE/build/src/test.c.o -c WORKSPACE/src/test.c
        output: CMakeFiles/slwjson.dir/src/test.c.o
Total Memory Usage: 544 MB
Browse Paths from compile_commands.json, from workspace folder: WORKSPACE
    WORKSPACE/src

Other Extensions

No response

Additional context

No response

Start with the C/C++ language server's IntelliSense handling for the c23 and gnu23 standards, using the example in WORKSPACE/src/test.c and the supplied clang 17 configuration. Reproduce the reported Problems, then verify that the C23 enum syntax, nullptr, and bool usage are accepted with diagnostics matching clang 17.