Update Clang Matrix (facebook#905)

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.

Also remove the job(s) constrained to the schedule event. They are just duplicative at this point.

Differential Revision: D68445686

.github/workflows/build_main.yml

@@ -60,9 +60,9 @@ 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"
@@ -90,25 +90,3 @@ jobs:
         name: redex-windows
         retention-days: 7
         path: build/Redex*.zip
-  build-deb_stable-w-clang-llvm-org_nightly:
-    if: github.event_name == 'schedule'
-    runs-on: ubuntu-latest
-    env:
-      CC: clang
-      CXX: clang++
-    strategy:
-      matrix:
-        clang_version:
-        - 0
-        - 15
-        - 17
-    steps:
-    - uses: actions/[email protected]
-    - uses: "./.github/actions/prepare_debian"
-      with:
-        install_clang_llvm_org: "${{ matrix.clang_version }}"
-    - uses: ./.github/actions/setup-build-and-test-w-make
-      with:
-        save_boost_cache: false
-        mode_32: false
-        job_name: "clang_upstream-${{ matrix.clang_version }}"