Run macos-12 CI build using Clang 15

This is to avoid build regressions when using the (at present:) latest
version of LLVM/Clang.

Fixes: #7547

Author: tautschnig

.github/workflows/pull-request-checks.yaml

@@ -535,7 +535,7 @@ jobs:
         run: |
           mkdir build
           cd build
-          cmake .. -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_C_COMPILER=/usr/bin/clang -DCMAKE_CXX_COMPILER=/usr/bin/clang++ -Dsat_impl=glucose
+          cmake .. -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_C_COMPILER=$(brew --prefix llvm@15)/bin/clang -DCMAKE_CXX_COMPILER=$(brew --prefix llvm@15)/bin/clang++ -Dsat_impl=glucose
       - name: Build with Ninja
         run: cd build; ninja -j3
       - name: Print ccache stats