Pull in r248379 from upstream clang trunk (by Jörg Sonnenberger):
Refactor library decision for -fopenmp support from Darwin into a function for sharing with other platforms.
Pull in r248424 from upstream clang trunk (by Jörg Sonnenberger):
Push OpenMP linker flags after linker input on Darwin. Don't add any libraries if -nostdlib is specified. Test.
Pull in r248426 from upstream clang trunk (by Jörg Sonnenberger):
Support linking against OpenMP runtime on NetBSD.
Pull in r250657 from upstream clang trunk (by Dimitry Andric):
Support linking against OpenMP runtime on FreeBSD.