Add option to update the compile_commands.json file instead of regenerating all of it

[guimafelipe]

When working in solutions with multiple projects, it is common to build just one specific project at a time and generate a log file for it. Users might want the possibility to update the already existing compile_commands.json changing only the entries for the files that are present in this log, leaving all the other files as they are.

For that, we could have a -u or --update flag that implements this behavior if the compile_commands.json already exists.

Usage:
ms2cc.exe --input-file <PATH_TO_MSBUILD.LOG> --source-directory <PATH_TO_PROJECT_SOURCE> --update

[guimafelipe]

As an example, here is the Powershell script I am currently using to implement this behavior:

$backupPath = $null

# If the -Update flag is provided, we want to update an existing compile_commands.json
# instead of overwriting it.
if ($Update) {
    # Save the old compile_commands.json if it exists in a new file
    $compileCommandsPath = Join-Path (Split-Path $PSScriptRoot -parent) "compile_commands.json"
    if (Test-Path $compileCommandsPath) {
        $backupPath = Join-Path (Split-Path $PSScriptRoot -parent) "compile_commands_old.json"
        Copy-Item -Path $compileCommandsPath -Destination $backupPath
        Write-Host "Backed up existing compile_commands.json to: $backupPath"
    } else {
        Write-Host "No existing compile_commands.json found to back up."
    }
}

& $ms2ccExe -i "temp-filtered2.log" -d (Split-Path $PSScriptRoot -parent) -p

Remove-Item "temp-filtered.log"
Remove-Item "temp-filtered2.log"

if ($Update -and (Test-Path $backupPath)) {
    # Merge the old and new compile_commands.json files
    $newCommands = Get-Content -Raw -Path (Join-Path (Split-Path $PSScriptRoot -parent) "compile_commands.json") | ConvertFrom-Json
    $oldCommands = Get-Content -Raw -Path $backupPath | ConvertFrom-Json

    # What defines the uniqueness of a command is the combination of "file" and "directory"
    # We want to merge the old and new commands, preferring the new ones in case of duplicates
    $mergedCommands = @{}
    foreach ($cmd in $oldCommands + $newCommands) {
        $key = "$($cmd.file)|$($cmd.directory)"
        $mergedCommands[$key] = $cmd
    }
    $mergedCommands.Values | ConvertTo-Json -Depth 10 | Set-Content -Path (Join-Path (Split-Path $PSScriptRoot -parent) "compile_commands.json") -Encoding utf8
    Write-Host "Merged old and new compile_commands.json files."

    Remove-Item $backupPath -Force
}