[OpenCL] opencl-c.h: refactor named addrspace builtins

The named address space overloads of builtins that take a pointer
argument are conditionalized on the `__opencl_c_generic_address_space`
feature macro (in a `#else` body).  Introduce an internal feature
macro instead, such that their availability can be controlled in a
single place and independently of the generic address space feature
macro.

This commit does not change the available builtins.

Differential Revision: https://reviews.llvm.org/D118158