Consider re-using our LLVM/Clang to build Rust

Rust re-builds LLVM, which takes a long time.

We could simply reuse the LLVM/Clang we build, and reduce the Rust compiling time, by adding -DLLVM_INSTALL_UTILS=ON to the options we use to compile Clang.

The only disadvantage is that we link everything to the updated libstdc++. On Rust we currently pass --enable-llvm-static-stdcpp. In my test I've removed it and defined -DLLVM_STATIC_LINK_CXX_STDLIB=ON on Clang, but it wasn't enough. As a workaround, we could either create a project that extracts libstdc++ from GCC, and provides it to projects that want to use Rust without a dependency on GCC (currently only cbindgen), or copy libstdc++ also to Rust.