Refactor how include paths are appended to the command arguments.

Summary:
This patch aims at removing redundancy in the way include paths for the regular and offloading toolchains are appended to the arguments list in the clang tool.

This was suggested by @rsmith in response to r275931.

Reviewers: rsmith, tra

Subscribers: rsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D22518

llvm-svn: 276929