Fix the numeric types used for array types

The type used internally in llvm for array bounds is uint64_t, but llvm-pretty was using Int32. #74 fixed this problem, but introduced a change that necessitated a major version bump.

Update DICompositeType for LLVM 7

This should be a non-breaking change if you don't construct or pattern-match on a DICompositeType.

Updates for LLVM 6

This release changed many datatypes in the AST related to debugging information (and their pretty-printers, accordingly). It is a breaking change for anyone pattern-matching on those types, but otherwise upgrading should be fairly straightforward.

The output of the pretty-printer is now compatible with more (later) versions of LLVM.