[OpenCL] Implemented restrictions for pointer conversions specified in OpenCL v2.0.

OpenCL v2.0 s6.5.5 restricts conversion of pointers to different address spaces:
- the named address spaces (__global, __local, and __private) => __generic - implicitly converted;
- __generic => named - with an explicit cast;
- named <=> named - disallowed;
- __constant <=> any other - disallowed.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@222834 91177308-0d34-0410-b5e6-96231b3b80d8