CI: switch FreeBSD to Clang 11 (similar to e2303f3378)

.ci/build-freebsd.sh

@@ -5,6 +5,13 @@
 # shellcheck disable=SC2046
 git submodule -q update --init --depth 1 $(awk '/path/ && !/llvm/ { print $3 }' .gitmodules)
 
+# Prefer newer Clang than in base system (see also .ci/install-freebsd.sh)
+# libc++ isn't in llvm* packages, so download manually
+fetch https://github.com/llvm/llvm-project/releases/download/llvmorg-11.0.0/libcxx-11.0.0.src.tar.xz
+tar xf libcxx-11.0.0.src.tar.xz
+export CC=clang11 CXX=clang++11
+export CXXFLAGS="$CXXFLAGS -nostdinc++ -isystem $PWD/libcxx-11.0.0.src/include"
+
 CONFIGURE_ARGS="
 	-DWITH_LLVM=OFF
 	-DUSE_PRECOMPILED_HEADERS=OFF

.ci/install-freebsd.sh

@@ -8,6 +8,9 @@ sed -i '' 's/quarterly/latest/' /etc/pkg/FreeBSD.conf
 export ASSUME_ALWAYS_YES=true
 pkg info # debug
 
+# Prefer newer Clang than in base system (see also .ci/build-freebsd.sh)
+pkg install llvm11
+
 # Mandatory dependencies (qt5-dbus and qt5-gui are pulled via qt5-widgets)
 pkg install git ccache cmake ninja qt5-qmake qt5-buildtools qt5-widgets qt5-concurrent glew openal-soft ffmpeg