[OpenCL] Add to_{global|local|private} builtin functions.

OpenCL builtin functions to_{global|local|private} accepts argument of pointer type to arbitrary pointee type, and return a pointer to the same pointee type in different addr space, i.e.

global gentype *to_global(gentype *p);
It is not desirable to declare it as

global void *to_global(void *);
in opencl header file since it misses diagnostics.

This patch implements these builtin functions as Clang builtin functions. In the builtin def file they are defined to have signature void*(void*). When handling call expressions, their declarations are re-written to have correct parameter type and return type corresponding to the call argument.

In codegen call to addr void *to_addr(void*) is generated with addrcasts or bitcasts to facilitate implementation in builtin library.

Differential Revision: http://reviews.llvm.org/D19932

llvm-svn: 270261