Expose "noduplicate" attribute as a property for intrinsics.

The "noduplicate" function attribute exists to prevent certain optimizations
from duplicating calls to the function. This is important on platforms where
certain function call duplications are unsafe (for example execution barriers
for CUDA and OpenCL).

This patch makes it possible to specify intrinsics as "noduplicate" and
translates that to the appropriate function attribute.

llvm-svn: 204200