[cuda] Added support for CUDA built-in variables.

Added cuda_builtin_vars.h which implements built-in CUDA variables
using __declattr(property).

Fields of built-in variables (except for warpSize) are implemented
using __declattr(property) which replaces read/write of a member field
with a call to a getter/setter member function, in this case with
appropriate NVPTX builtin.

Added a test case to check diagnostics on attempt to construct or
improperly access a built-in variable.

Differential Revision: http://reviews.llvm.org/D9064

llvm-svn: 235398