Support SDCC as compiler

Type: LanguageService

This issue is closely related to #2499 but more general. The linked issue talks requests that VSCode supports some of SDCC compiler's syntax for Intel 8051 projects. This issue requests that SDCC is generally a supported and recognized compiler in VSCode's C/C++ extension, since SDCC can do much more (compile code for STM8, Z80, PIC14, PIC16, etc.).

Describe the bug

• OS and Version: Windows 10 x64
• VS Code Version: 1.54.1
• C/C++ Extension Version: 1.2.2
• Other extensions you installed (and if the issue persists after disabling them): PlatformIO (2.3.0)
• Does this issue involve using SSH remote to run the extension on a remote machine?: No
• A clear and concise description of what the bug is, including information about the workspace (i.e. is the workspace a single project or multiple projects, size of the project, etc).:
  • When using SDCC as the compiler in the compilerPath of the .vscode/c_cpp_properties.json file, it is not being recongized correctly with the message "Unable to resolve configuration with compilerPath [..]/sdcc.exe. Using cl.exe instead."

Steps to reproduce

1. (Note: I'm using PlatformIO to generate a project, auto-download SDCC and auto-generate the c_cpp_properties.json that is causing issues)
2. Install the PlatformIO extension
3. Create a new project for the STM8Blue board and SPL as framework
4. Switch to the newly created project
5. Change the platformio.ini of the project to

[env:stm8sblue]
platform = ststm8
board = stm8sblue

6. Press Ctrl+Shift+P -> Rebuild IntelliSense
7. Observe generated .vscode/c_cpp_properties.json containing SDCC in the compilerPath, example:

//
// !!! WARNING !!! AUTO-GENERATED FILE!
// PLEASE DO NOT MODIFY IT AND USE "platformio.ini":
// https://docs.platformio.org/page/projectconf/section_env_build.html#build-flags
//
{
    "configurations": [
        {
            "name": "PlatformIO",
            "includePath": [
                "C:/Users/Max/Documents/PlatformIO/Projects/stm8_testing/include",
                "C:/Users/Max/Documents/PlatformIO/Projects/stm8_testing/src",
                "C:/Users/Max/.platformio/packages/tool-unity",
                ""
            ],
            "browse": {
                "limitSymbolsToIncludedHeaders": true,
                "path": [
                    "C:/Users/Max/Documents/PlatformIO/Projects/stm8_testing/include",
                    "C:/Users/Max/Documents/PlatformIO/Projects/stm8_testing/src",
                    "C:/Users/Max/.platformio/packages/tool-unity",
                    ""
                ]
            },
            "defines": [
                "F_CPU=16000000L",
                "PLATFORMIO=50101",
                "STM8S_BLUE",
                "STM8S103",
                ""
            ],
            "compilerPath": "C:/Users/Max/.platformio/packages/toolchain-sdcc/bin/sdcc.exe",
            "compilerArgs": [
                "-mstm8",
                ""
            ]
        }
    ],
    "version": 4
}

Expected behavior

The SDCC compiler is recongized correctly in VSCode and compiler-specific includes like `#include "mcs51/8051.h" are recongized, and special SDCC syntax for e.g. special function registers (SFRs) and interrupt definitions are recongized.

1. Logs from the command C/C++: Log Diagnostics: None relevant -- see below
2. Logs from the language server: None relevant: see below

Actual behavior

Error message in the "C/C++ Configuration warnings" output tab:

[10.3.2021, 20:14:57] Die Konfiguration mit compilerPath "C:/Users/Max/.platformio/packages/toolchain-sdcc/bin/sdcc.exe" kann nicht aufgelöst werden. Stattdessen wird "cl.exe" verwendet.

Translates to

Unable to resolve configuration with compilerPath [..]/sdcc.exe. Using cl.exe instead."

Due to that of course, nothing SDCC specific like the compiler-specific includes or syntaxes are displayed correctly in VSCode.

Screenshots
None relevant

Reproduce the issue with the provided PlatformIO STM8 project and generated .vscode/c_cpp_properties.json, focusing on compilerPath handling for sdcc.exe. Trace how the C/C++ extension identifies compilers and applies compiler-specific includes and syntax. Done means SDCC is recognized without falling back to cl.exe and the reported SDCC headers and syntax are handled correctly.