CI: update clang to v18 (#13937)

Signed-off-by: Philip H <47042125+pheiduck@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

Author: pheiduck

.github/workflows/ci.yml

@@ -22,7 +22,7 @@ jobs:
     env:
       CC: ${{ matrix.compiler }}
       GCC_VER: 13
-      CLANG_VER: 17
+      CLANG_VER: 18
       TEST: test
       SRCDIR: ./src
       LEAK_CFLAGS: -DEXITFREE