[OpenMP] Bugfix: output file name drops the absolute path where full path is needed.

Summary: When composing the output file name, the path to the file is being dropped. The full path is required.

Reviewers: Hahnfeld, ABataev, caomhin, carlo.bertolli, hfinkel, tra

Reviewed By: tra

Subscribers: hfinkel, tra, cfe-commits

Differential Revision: https://reviews.llvm.org/D37912

llvm-svn: 314156