Port to LLVM 3.5

This includes all changes needed for our own plugin to compile against LLVM 3.5. Unfortunately `iiglue` does not yet support LLVM 3.5, so we cannot simply switch over to that and forget about 3.4. But once `iiglue` catches up to 3.5, this should be everything we need in order to use that too.

For now, these changes do come with a few other benefits:

* You can switch to a different LLVM installation by setting `LLVM_CONFIG` on the `scons` command line.
* Any configuration changes made on the `scons` command line (such as `IIGLUE=...` or `LLVM_CONFIG=...`) will be automatically remembered for future `scons` invocations.
* `opt` (from LLVM) is now properly included among the implicit dependencies of `iiglue`-generated outputs.