[AttrBuilder] Make handling of type attributes more generic (NFCI)

While working on the elementtype attribute, I felt that the type
attribute handling in AttrBuilder is overly repetitive. This patch
converts the separate Type* members into an std::array<Type*>, so
that all type attribute kinds can be handled generically.

There's more room for improvement here (especially when it comes to
converting the AttrBuilder to an Attribute), but this seems like a
good starting point.

Differential Revision: https://reviews.llvm.org/D105658