[OpenCL] Defines helper function for OpenCL default address space

Helper function `getDefaultOpenCLPointeeAddrSpace()` introduced to
`ASTContext` class. It returns default OpenCL address space
depending on language version and enabled features. If generic
address space is supported, the helper function returns value
`LangAS::opencl_generic`. Otherwise, value `LangAS::opencl_private`
is returned. Code refactoring changes performed in several suitable
places.

Differential Revision: https://reviews.llvm.org/D109874