Allow clang to compile the "extern" storage class in OpenCL 1.2. 
The tests (forgot to svn add, sorry!).

Patch from Fraser Cormack!



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199907 91177308-0d34-0410-b5e6-96231b3b80d8