All the parts needed to support wasm are included in upstream clang, lld, and compiler-rt, as of the LLVM 8.0 release. However, to use it, you'll need to build WebAssembly-targeted versions of the library parts, and it can be tricky to get all the CMake invocations lined up properly.
To make things easier, we provide prebuilt packages that provide builds of Clang and sysroot libraries.
WASI doesn't yet support
longjmp or C++ exceptions, as it is
waiting for unwinding support in WebAssembly.