[C++4OpenCL] Introduces __remove_address_space utility

This change provides a way to conveniently declare types that have
address space qualifiers removed.

Since OpenCL adds address spaces implicitly even when they are not
specified in source, it is useful to allow deriving address space
unqualified types.

Fixes llvm.org/PR45326

Differential Revision: https://reviews.llvm.org/D106785