[OpenCL] Handle address space conversions for constexpr (PR44177)

The AST for the constexpr.cl test contains address space conversion
nodes to cast through the implicit generic address space.  These
caused the evaluator to reject the input as constexpr in C++ for
OpenCL mode, whereas the input was considered constexpr in plain C++
mode as the AST won't have address space cast nodes then.

Fixes PR44177.

Differential Revision: https://reviews.llvm.org/D71015