[langref] attempt to clarify semantics of inttoptr/ptrtoint for non-integral types

In review discussion on D104322, Eli and Roman quite reasonable raised concerns about the LangRef not really providing a precise definition for inttoptr/ptrtoint on non-integral types. These had previously been disallowed, but I'd pragmatically allowed them in ac81cb7e. This is my attempt to improve the situation.

Differential Revision: https://reviews.llvm.org/D104547