Fix build with LLVM top-of-tree, fix warnings, remove LLVM 3.0 support

There are three commits here.

1. A whole bunch of header files have moved and some APIs have changed in the new llvm 3.3 development branch.  These changes get ispc building again.

2. The second one just fixes some compiler warnings (all cosmetic)

3. The third removes support for building with LLVM 3.0.  You may or may not want to take this, but it cleans up the code in a few places and generally removes cruft.  Now that LLVM 3.2 has been released, it seems reasonable to demand LLVM 3.1 or higher, especially since 3.0's support for AVX is much less good than 3.1 and 3.2