Image types were previously available, but not working. This patch
adds image handling for OpenCL builtin functions.
Image types must have an access specifier (read_only, etc) when used.
This depends on:
-First, splitting opencl-c.h file: https://reviews.llvm.org/D63256/new/
-Second, Adding generic types: https://reviews.llvm.org/D63434
-Third, Adding const, volatile and pointer types: https://reviews.llvm.org/D63442