Certain multi-platform languages, such as OpenCL, have the concept of
address spaces which is both (1) a "semantic" concept and
(2) possibly a hardware level restriction. It is desirable to
be able to discard/merge the LLVM-level address spaces on arguments for which
there is no difference to the current backend while keeping
track of the semantic address spaces in a funciton prototype. To do this
enable addition of the address space into the name-mangling process. Add
some tests to document this behaviour against inadvertent changes.

Patch by Michele Scandale!