Update Clang Matrix

Summary:
The LLVM project no longer distributes Clang-15.

At the time of writing, the default being installed (which we call `0`) is Clang-18. Also available are Clang-17 and Clang-19.

Replace Clang-15 with Clang-19.

Differential Revision: D68445686

.github/workflows/build_main.yml

@@ -99,9 +99,9 @@ jobs:
     strategy:
       matrix:
         clang_version:
-        - 0
-        - 15
+        - 0  # At time of edit 18.
         - 17
+        - 19
     steps:
     - uses: actions/[email protected]
     - uses: "./.github/actions/prepare_debian"