Set default llvm version to 15

[serkm]

No description provided.

[b-studios]

One test is failing for me locally: effekt.LLVMTests.examples/llvm/forking.effekt.

[b-studios]

==> X effekt.LLVMTests.examples/llvm/forking.effekt (llvm)  0.306s java.lang.RuntimeException: Nonzero exit value: 134
    at scala.sys.process.ProcessBuilderImpl$AbstractBuilder.slurp(ProcessBuilderImpl.scala:164)
    at scala.sys.process.ProcessBuilderImpl$AbstractBuilder.$bang$bang(ProcessBuilderImpl.scala:121)
    at effekt.Driver.evalLLVM(Driver.scala:155)
    at effekt.Driver.evalLLVM$(Driver.scala:25)
    at effekt.LLVMTests$$anon$1.evalLLVM(LLVMTests.scala:31)
    at effekt.Driver.eval(Driver.scala:109)
    at effekt.Driver.eval$(Driver.scala:25)
    ```

[serkm]

CI on the main branch fails due to the install script switching to llvm 16.
I've changed it to install version 15 explicitly.

[serkm]

@b-studios honestly, I don't know what could have caused that failure.
Version 12 shouldn't work at all at this point, and if 15 is already in use, this shouldn't change anything.

[b-studios]

Ok, let's just merge this and I'll try to figure it out on my end.