Support ${relativeFileDirname} in c_cpp_properties.json

Type: LanguageService

Describe the bug

• OS and Version: Windows 10
• VS Code Version: 1.45.1
• C/C++ Extension Version: 0.28.2
• Other extensions you installed (and if the issue persists after disabling them): N/A
• Does this issue involve using SSH remote to run the extension on a remote machine?: No

VSCode supports ${relativeFileDirname} in launch.json and tasks.json. See here. I would like to have support for this variable in c_cpp_properties.json. I have a module testing framework that generates mocked functions and places them in mock_defaults.h. Each test has its own version of this header. When the test is open in the editor cpp_tools should grab ${relativeFileDirname} and use it for that translation units include paths.

Ex c_cpp_properties.json:

{
    "configurations": [
        {
            "defines": [
                "${default}"
            ],
            "includePath": [
                "${default}",
                "${workspaceFolder:System Code}/bin/mod/${relativeFileDirname}"
            ],
            "intelliSenseMode": "msvc-x64",
            "name": "Win32"
        }
    ],
    "version": 4
}

Expected behavior
The above config should allow each translation unit (i.e. module test in my case) to use a path unique to itself.

For expample,
${workspaceFolder} = c/code/dev
${fileDirname} = c/code/dev/tests/path/to/test
${relativeFileDirname} = tests/path/to/test
final include path = c/code/dev/bin/mod/tests/path/to/test

Start with the c_cpp_properties.json variable expansion behavior and compare it with the documented variable support in launch.json and tasks.json. Use the example configuration and path values in this issue to verify that ${relativeFileDirname} resolves per translation unit and produces the expected include path.