Enable support for switching between different output targets with compile_commands.json

    Hi @sean-mcmanus 

Here is a minimal example of this scenario

main.cpp

#include <iostream>

void func1()
{
#ifdef TARGET_1
    std::cout << "Target 1" << std::endl;
#elif defined(TARGET_2)
    std::cout << "Target 2" << std::endl;
#else
    std::cout << "Other" << std::endl;
#endif
}

int main()
{
    func1();
    return 0;
}

c_cpp_properties.json

{
    "configurations": [
        {
            "name": "Linux",
            "includePath": [
                "${workspaceFolder}/**"
            ],
            "defines": [],
            "compilerPath": "/bin/g++",
            "cStandard": "c11",
            "cppStandard": "c++17",
            "intelliSenseMode": "linux-gcc-x64",
            "compileCommands": ".vscode/compile_commands.json"
        }
    ],
    "version": 4
}

compile_commands.json

[
    {
        "file": "/home/karim/test/main.cpp",
        "directory": "/home/karim/test/build",
        "arguments": [
            "g++",
            "-c",
            "-o",
            "b.o",
            "-DTARGET_2",
            "/home/karim/test/main.cpp"
        ],
        "output": "/home/karim/test/build/b.o"
    },
    {
        "file": "/home/karim/test/main.cpp",
        "directory": "/home/karim/test/build",
        "arguments": [
            "g++",
            "-c",
            "-o",
            "a.o",
            "-DTARGET_1",
            "/home/karim/test/main.cpp"
        ],
        "output": "/home/karim/test/build/a.o"
    }
]

Note that "output" field is used to distinguish between different processing modes (output file) of the same source file as documented by the Json Compilation Database Specification

Originally posted by @KarimAshraf1995 in https://github.com/microsoft/vscode-cpptools/discussions/10408#discussioncomment-4893606

Start by tracing how compile_commands.json entries are selected for a source file, using the two entries for main.cpp and their output fields as the behavioral example. Check how c_cpp_properties.json connects the compilation database to IntelliSense. Done means the extension can distinguish and switch between the TARGET_1 and TARGET_2 processing modes instead of treating one command as canonical.