ci: add support for LLVM 15

This commit switches to LLVM 15 everywhere by default, while still
keeping LLVM 14 support.

.github/workflows/windows.yml

@@ -35,7 +35,7 @@ jobs:
         uses: actions/cache@v3
         id: cache-llvm-source
         with:
-          key: llvm-source-14-windows-v2
+          key: llvm-source-15-windows-v1
           path: |
             llvm-project/clang/lib/Headers
             llvm-project/clang/include
@@ -49,7 +49,7 @@ jobs:
         uses: actions/cache@v3
         id: cache-llvm-build
         with:
-          key: llvm-build-14-windows-v2
+          key: llvm-build-15-windows-v1
           path: llvm-build
       - name: Build LLVM
         if: steps.cache-llvm-build.outputs.cache-hit != 'true'