[Driver][CUDA][OpenMP] Reimplement tool selection in the driver.

Summary:
This creates a tool selector in the driver that replaces the existing one. The goal is to better organize the code and make the selector easier to scale, in particular in the presence of offload actions that can be collapsed. 

The current implementation became more confusing when the support for offloading actions was added. This concern was expressed by Eric in http://reviews.llvm.org/D9888.

This patch does not add new testing, it preserves the existing functionality.

Reviewers: echristo, tra, jlebar, rsmith, ABataev, hfinkel

Subscribers: whchung, guansong, mkuron, mehdi_amini, cfe-commits, Hahnfeld, andreybokhanko, caomhin, arpith-jacob, carlo.bertolli

Differential Revision: https://reviews.llvm.org/D21840

llvm-svn: 285307