[OpenMP] Enable executable lookup into driver directory.

Summary: Invoking the compiler inside a script causes the clang-offload-bundler executable to not be found. This patch enables the lookup for executables in the driver directory where the clang-offload-bundler resides.

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

Reviewed By: hfinkel

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D36537

llvm-svn: 310513