Use only llvm::Constant in the interface instead.

Bug swiftshader:10

Change-Id: Ideb2ed1ee083e4e6896c4bc8e8830ea02972b487
Reviewed-on: https://swiftshader-review.googlesource.com/7273Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>