Remove the notion of primitive types.

They were out of place since the introduction of arbitrary precision integer
types.

This also synchronizes the documentation to Types.h, so it refers to first class
types and single value types.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196661 91177308-0d34-0410-b5e6-96231b3b80d8