[cuda] Allow using integral non-type template parameters as launch_bounds attribute arguments.

 - Changed CUDALaunchBounds arguments from integers to Expr* so they can
   be saved in AST for instantiation.
 - Added support for template instantiation of launch_bounds attrubute.
 - Moved evaluation of launch_bounds arguments to NVPTXTargetCodeGenInfo::
   SetTargetAttributes() where it can be done after template instantiation.
 - Added a warning on negative launch_bounds arguments.
 - Amended test cases.

Differential Revision: http://reviews.llvm.org/D8985

llvm-svn: 235452