ClangCodeModel: Use more suitable location for compile_commands.json

... for clangd.
Putting it in the build directory seems sensible in principle, but that
can be problematic for in-source builds. So introduce another level of
nesting to prevent conflicts.

Fixes: QTCREATORBUG-26431
Change-Id: Id66aa0852d206695f2fc2ec42292b1cecefe2b59
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangcodemodelplugin.cpp

@@ -82,7 +82,7 @@ void ClangCodeModelPlugin::generateCompilationDB()
 
     QFuture<GenerateCompilationDbResult> task
             = QtConcurrent::run(&Internal::generateCompilationDB, projectInfo,
-                                CompilationDbPurpose::Project,
+                                projectInfo->buildRoot(), CompilationDbPurpose::Project,
                                 warningsConfigForProject(target->project()),
                                 optionsForProject(target->project()));
     Core::ProgressManager::addTask(task, tr("Generating Compilation DB"), "generate compilation db");