[OpenCL] Generic address space has been added in OpenCL v2.0.

To support it in the frontend, the following has been added:  
- generic address space type attribute;
- documentation for the OpenCL address space attributes;
- parsing of __generic(generic) keyword;
- test code for the parser and diagnostics.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@222831 91177308-0d34-0410-b5e6-96231b3b80d8