LLVM backend using headers not available on Windows

I tried running the benchmarks from `examples/benchmarks` with the LLVM backend on Windows inside WSL, and ran into some issues. After being prompted to install clang
```sudo apt install clang```

I tried it again. Then another error greets me:
```
./out/bounce.ll:5:21: warning: ptr type is only supported in -opaque-pointers mode
%Environment = type ptr
^
./out/bounce.ll:5:21: error: expected type
%Environment = type ptr
^
1 error generated.
Exception in thread "main" java.lang.RuntimeException: Nonzero exit value: 1
at scala.sys.process.ProcessBuilderImpl$AbstractBuilder.slurp(ProcessBuilderImpl.scala:164)
at scala.sys.process.ProcessBuilderImpl$AbstractBuilder.$bang$bang(ProcessBuilderImpl.scala:121)
at effekt.Runner.exec(Runner.scala:136)
at effekt.Runner.exec$(Runner.scala:14)
at effekt.LLVMRunner$.exec(Runner.scala:281)
...
```
Any help is appreciated.

Start by reproducing the benchmark from examples/benchmarks in WSL and inspect the generated out/bounce.ll. Then trace the failure through effekt.Runner and LLVMRunner.exec; done means the LLVM backend benchmarks compile successfully with the available Windows/WSL toolchain.

Written by the indexing model from the issue text.