[OpenCL] Allow addr space spelling without __ prefix in C++.

For backwards compatibility we allow alternative spelling of address
spaces - 'private', 'local', 'global', 'constant', 'generic'.

In order to accept 'private' correctly, parsing has been changed to
understand different use cases - access specifier vs address space.

Fixes PR40707 and PR41011!

Differential Revision: https://reviews.llvm.org/D59603

llvm-svn: 356888