drm_hwcomposer: CI: Bump-up clang toolchain to v15

Address new clang-tidy findings, in most cases 'misc-const-correctness'
check was addressed by adding 'const' modifier, or in some cases changed
to 'auto' (where it's better for formatting).

Signed-off-by: Roman Stratiienko <r.stratiienko@gmail.com>
diff --git a/tests/worker_test.cpp b/tests/worker_test.cpp
index d1eb2b3..ac0c5fc 100644
--- a/tests/worker_test.cpp
+++ b/tests/worker_test.cpp
@@ -17,7 +17,7 @@
   void Routine() override {
     Lock();
     if (!enabled_) {
-      int ret = WaitForSignalOrExitLocked();
+      auto ret = WaitForSignalOrExitLocked();
       if (ret == -EINTR) {
         Unlock();
         return;