[OpenCL] Fix access qualifiers handling for typedefs

OpenCL s6.6: "Access qualifier must be used with image object arguments
of kernels and of user-defined functions [...] If no qualifier is
provided, read_only is assumed".

This does not define the behavior for image types used in typedef
declaration, but following the spec logic, we should allow access
qualifiers specification in typedefs, e.g.:

  typedef write_only image1d_t img1d_wo;

Unlike cv-qualifiers, user cannot add access qualifier to a typedef
type, i.e. this is not allowed:

  typedef image1d_t img1d; // note: previously declared 'read_only' here
  void foo(write_only img1d im) {} // error: multiple access qualifier

Patch by Andrew Savonichev.
Reviewers: Anastasia Stulova.

Differential revision: http://reviews.llvm.org/D20948

llvm-svn: 274858