Use LLVM 3.2 by default

This pull request makes LLVM 3.2 the default, recommended version of LLVM. This applies to the library we use to build the generator. It is not related to which compiler is used.

LLVM 3.1 can still be used. The build scripts and the source code are compatible with both 3.1 and 3.2.

We make LLVM 3.2 the default because 3.1 does not work on Windows.