[OpenMP] Accept shortened triples for -Xopenmp-target=

This patch builds on the change in D117634 that expanded the short
triples when passed in by the user. This patch adds the same
functionality for the `-Xopenmp-target=` flag. Previously it was
unintuitive that passing `-fopenmp-targets=nvptx64
-Xopenmp-target=nvptx64 <arg>` would not forward the arg because the
triples did not match on account of `nvptx64` being expanded to
`nvptx64-nvidia-cuda`.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D118495