[Clang] Use private address space for builtin_alloca return type for OpenCL (#95750)

The __builtin_alloca was returning a flat pointer with no address space
when compiled using openCL1.2 or below but worked fine with openCL2.0
and above. This accounts to the fact that later uses the concept of
generic address space which supports cast to other address space(i.e to
private address space which is used for stack allocation) .

But, in actuality, as it returns pointer to the stack, it should be
pointing to private address space irrespective of openCL version becuase
builtin_alloca allocates stack memory used for current function in which
it is called. Thus,it requires redefintion of the builtin function with
appropraite return pointer to private address space.