Treat warnings as errors in out of tree CI builds

Ensure build warnings are not left unnoticed in CI builds.  Only do
this for out of tree builds, such that any warnings from compiling
LLVM sources are not breaking the build unnecessarily.

Author: svenvh

.github/workflows/check-out-of-tree-build.yml

@@ -64,6 +64,7 @@ jobs:
           mkdir build && cd build
           cmake ${{ github.workspace }} \
             -DCMAKE_BUILD_TYPE=${{ matrix.build_type }} \
+            -DCMAKE_CXX_FLAGS="-Werror" \
             -DLLVM_INCLUDE_TESTS=ON \
             -DLLVM_EXTERNAL_LIT="/usr/lib/llvm-${{ env.LLVM_VERSION }}/build/utils/lit/lit.py" \
             -G "Unix Makefiles"