[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