[OpenCL] Make generic addrspace optional for -fdeclare-opencl-builtins

Currently, -fdeclare-opencl-builtins always adds the generic address
space overloads of e.g. the vload builtin functions in OpenCL 3.0
mode, even when the generic address space feature is disabled.

Guard the generic address space overloads by the
`__opencl_c_generic_address_space` feature instead of by OpenCL
version.

Guard the private, global, and local overloads using the internal
`__opencl_c_named_address_space_builtins` feature.

Differential Revision: https://reviews.llvm.org/D107769