[OpenCL] Fix size of image type

The size of image type is reported incorrectly as size of a pointer to address space 0, which causes error when casting image type to pointers by __builtin_astype.

The fix is to get image address space from TargetInfo then report the size accordingly.

Differential Revision: https://reviews.llvm.org/D22927

llvm-svn: 277647
6 files changed