[NVPTX] Run clang-format on all NVPTX sources.

Hopefully this resolves any outstanding style issues and gives us
an automated way of ensuring we conform to the style guidelines.
[NVPTX] Add NVVMReflect pass to allow compile-time selection of
specific code paths.

This allows us to write code like:

  if (__nvvm_reflect("FOO"))
    // Do something
  else
    // Do something else

and compile into a library, then give "FOO" a value at kernel
compile-time so the check becomes a no-op.
[NVPTX] Remove support for SM < 2.0.  This was never fully supported anyway.