Improve support for Float types

[kquick]

There is currently support lacking for all of the different Float types (e.g. at https://github.com/elliottt/llvm-pretty/blob/master/src/Text/LLVM/AST.hs#L346). Work should be done to add more complete FloatType support.

[langston-barrett]

@kquick That link may now point to a different line than originally intended. Do you happen to recall what aspect of supporting more float types you had in mind?