Add support for CUDA __launch_bounds__ attribute to CodeGen.

Sema does have a CUDALaunchBoundsAttr, but CodeGen was doing nothing with it.
This change translates CUDALaunchBoundsAttr to maxntidx and minctasm
metadata, which NVPTX then translates to the correct PTX directives.

Patch by Manjunath Kudlur.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206302 91177308-0d34-0410-b5e6-96231b3b80d8