[NVPTX] Read __CUDA_FTZ from module flags in NVVMReflect.

Summary:
Previously the NVVMReflect pass would read its configuration from
command-line flags or a static configuration given to the pass at
instantiation time.

This doesn't quite work for clang's use-case.  It needs to pass a value
for __CUDA_FTZ down on a per-module basis.  We use a module flag for
this, so the NVVMReflect pass needs to be updated to read said flag.

Reviewers: tra, rnk

Subscribers: cfe-commits, jholewinski

Differential Revision: http://reviews.llvm.org/D18672

llvm-svn: 265090