github/workflow: use clang version 12

Clang's ThreadSanitizer in version 10 and 11 emits a false positive
when using condition variables ("WARNING: ThreadSanitizer: double lock
of a mutex").
This was fixed in version 12 (google/sanitizers#1259).

.github/workflows/build-and-test.yml

@@ -11,6 +11,12 @@ jobs:
     env:
       CXX: ${{ matrix.compiler }}
     steps:
+    - name: Install clang++-12
+      if: ${{ matrix.compiler == 'clang++' }}
+      uses: KyleMayes/install-llvm-action@v1.3.0
+      with:
+        version: "12.0.0"
+        directory: ${{ runner.temp }}/llvm
     - name: Info
       run: cmake --version && $CXX --version && uname -a
     - uses: actions/checkout@v2