[OpenMP] Expand short verisions of OpenMP offloading triples

The OpenMP offloading libraries are built with fixed triples and linked
in during compile time. This would cause un-helpful errors if the user
passed in the wrong expansion of the triple used for the bitcode
library. because we only support these triples for OpenMP offloading we
can normalize them to the full verion used in the bitcode library.

Reviewed By: jdoerfert, JonChesterfield

Differential Revision: https://reviews.llvm.org/D117634