oss-fuzz: build our own libc++-16

oss-fuzz comes with LLVM15-ish, but that (aside from macOS,
which comes with LLVM16), is the only libc++-based distro,
so it looks like we can get away with requiring libc++-16.

It is a pity that we have to support clang-15,
but that is much more invasive of a change.

Refs. google/oss-fuzz#9989

.ci/oss-fuzz.sh

@@ -22,6 +22,26 @@ export CMAKE_GENERATOR=Ninja
 
 ln -f -s /usr/local/bin/lld /usr/bin/ld
 
+cd "$SRC"
+
+wget -q https://github.com/llvm/llvm-project/releases/download/llvmorg-16.0.6/llvm-project-16.0.6.src.tar.xz
+tar -xf llvm-project-16.0.6.src.tar.xz llvm-project-16.0.6.src/{runtimes,cmake,llvm/cmake,libcxx,libcxxabi}/
+LIBCXX_BUILD="$SRC/llvm-project-16.0.6.build"
+mkdir "$LIBCXX_BUILD"
+cmake -S llvm-project-16.0.6.src/runtimes/ -B "$LIBCXX_BUILD" \
+      -DCMAKE_BUILD_TYPE=Release \
+      -DBUILD_SHARED_LIBS=OFF \
+      -DLLVM_INCLUDE_TESTS=OFF \
+      -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi" \
+      -DLIBCXX_ENABLE_SHARED=OFF \
+      -DLIBCXX_ENABLE_STATIC_ABI_LIBRARY=ON \
+      -DLIBCXXABI_ENABLE_SHARED=OFF \
+      -DLIBCXX_INCLUDE_BENCHMARKS=OFF \
+      -DLIBCXXABI_ADDITIONAL_COMPILE_FLAGS="-fno-sanitize=vptr"
+cmake --build "$LIBCXX_BUILD" -- -j$(nproc) cxx cxxabi
+
+CXXFLAGS="$CXXFLAGS -nostdinc++ -nostdlib++ -isystem $LIBCXX_BUILD/include -isystem $LIBCXX_BUILD/include/c++/v1 -L$LIBCXX_BUILD/lib -lc++ -lc++abi"
+
 if [[ $SANITIZER = *undefined* ]]; then
   CFLAGS="$CFLAGS -fsanitize=unsigned-integer-overflow -fno-sanitize-recover=unsigned-integer-overflow"
   CXXFLAGS="$CXXFLAGS -fsanitize=unsigned-integer-overflow -fno-sanitize-recover=unsigned-integer-overflow"