[OpenCL] Fix default address space in template argument deduction.

When deducing a reference type for forwarding references prevent
adding default address space of a template argument if it is given.

This got reported in PR48896 because in OpenCL all parameters are
in private address space and therefore when we initialize a
forwarding reference with a parameter we should just inherit the
address space from it i.e. keep __private instead of __generic.

Tags: #clang

Differential Revision: https://reviews.llvm.org/D95624