[OpenCL] Reduce duplicate defs by using multiclasses; NFC

Builtin definitions with pointer arguments were duplicated to provide
overloads differing in the pointer argument's address space.

Reduce this duplication by capturing the definitions in multiclasses.
This still results in the same number of builtins in the generated
tables, but the description is more concise now.

Differential Revision: https://reviews.llvm.org/D107151