[OpenMP] Don't build the offloading driver without a source input

[OpenMP] Don't build the offloading driver without a source input

The Clang driver additional stages to build a complete offloading
program for applications using CUDA or OpenMP offloading. This normally
requires either a source file input or a valid object file to be
handled. This would cause problems when trying to compile an assembly or
LLVM IR file through clang with flags that would enable offloading. This
patch simply adds a check to prevent the offloading toolchain from being
used if we don't have a valid source file.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D125705