Use newer LLVM version

Author: nilsfriess

.github/workflows/linux.yml

@@ -7,7 +7,7 @@ jobs:
     name: Run tests with hipSYCL
     strategy:
       matrix:
-        clang_version: [13]
+        clang_version: [15]
         rocm_version: ['5.1.1']
         cuda_version: ['11.6']
         os: [ubuntu-20.04]