[libomptarget] Split implementation of interface functions

This last of four patches adds a new file for the interface
functions that Clang uses during code generation. The only
change except simply moving the current code is renaming the
function CheckDeviceAndCtors() and using the correct type for
64bit device ids.

Differential Revision: https://reviews.llvm.org/D40801

llvm-svn: 319972