[OpenCL] Add missing address spaces in IR generation of blocks

Modify ObjC blocks impl wrt address spaces as follows:

- keep default private address space for blocks generated
as local variables (with captures);

- add global address space for global block literals (no captures);

- make the block invoke function and enqueue_kernel prototype with
the generic AS block pointer parameter to accommodate both 
private and global AS cases from above;

- add block handling into default AS because it's implemented as
a special pointer type (BlockPointer) in the frontend and therefore
it is used as a pointer everywhere. This is also needed to accommodate
both private and global AS blocks for the two cases above.

- removes ObjC RT specific symbols (NSConcreteStackBlock and
NSConcreteGlobalBlock) in the OpenCL mode.

Review: https://reviews.llvm.org/D28814
llvm-svn: 293286