[OpenCL 2.0] Apply default address space (AS).

If AS of a variable/parameter declaration is not set by the source,
OpenCL v2.0 s6.5 defines explicit rules for default ASes:

- The AS of global and local static variables defaults to global;
- All pointers point to generic AS.

http://reviews.llvm.org/D13168

llvm-svn: 253863