[OpenMP] Replace offloading option that start with -o with -fo.

Summary:
The current offloading implementation is using -omptargets and -omp-host-ir-file-path options in the frontend. This causes the user a lot of trouble due to to the conflicts with the -o option. E.g. if the user misspells  omptargets he will end up with a file with a weird name.

This patches replaces these two options with  -fomptargets and -fomp-host-ir-file-path to avoid these issues, and it is also more consistent with the other options like -fopenmp.

Reviewers: hfinkel, carlo.bertolli, arpith-jacob, kkwli0, ABataev

Subscribers: cfe-commits, caomhin, fraggamuffin

Differential Revision: http://reviews.llvm.org/D18112

llvm-svn: 263442