[OpenCL] Diagnose redundant address space conversion

Add a warning if a parameter with a named address space is passed
to a to_addr builtin.

For example:

  int i;
  to_private(&i); // generate warning as conversion from private to private is redundant.

Patch by Alistair Davies.

Differential Revision: https://reviews.llvm.org/D51411

llvm-svn: 342638