[AMDGPU] Give enum an explicit 64-bit type to fix MSVC 2013 failures

Recall that MSVC always gives enums the type 'int', nothing else.  MSVC
2015 does not appear to have this problem anymore.

Clang-cl -Wmicrosoft-enum-value flags this, FWIW, so now I have a true
positive for my warning. :)

llvm-svn: 278762