Revert "Revert "Use QtcProcess for clangbackend""

This reverts commit d65e47b5c6.
Reverts the quickfix for the performance regression of code completion.
Followed by a minimal fix.

Task-number: QTCREATORBUG-26829
Change-Id: I1afe71c24ee94caadd4fa849c29f6fc030fb1a8b
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>
Reviewed-by: hjk <hjk@qt.io>

tests/unit/unittest/unittests-main.cpp

@@ -29,11 +29,13 @@
 #include <sqlitelibraryinitializer.h>
 
 #include <sqliteglobal.h>
-#include <utils/temporarydirectory.h>
+#include <utils/launcherinterface.h>
 #include <utils/singleton.h>
+#include <utils/temporarydirectory.h>
 
 #include <QGuiApplication>
 #include <QLoggingCategory>
+#include <QScopeGuard>
 
 #ifdef WITH_BENCHMARKS
 #include <benchmark/benchmark.h>
@@ -59,6 +61,8 @@ int main(int argc, char *argv[])
     Sqlite::Database::activateLogging();
 
     QGuiApplication application(argc, argv);
+    Utils::LauncherInterface::setPathToLauncher(qApp->applicationDirPath() + '/'
+                                                + QLatin1String(TEST_RELATIVE_LIBEXEC_PATH));
     testing::InitGoogleTest(&argc, argv);
 #ifdef WITH_BENCHMARKS
     benchmark::Initialize(&argc, argv);