[Driver] Set the default driver mode based on the executable.

Currently, if --driver-mode is not passed at all, it will default
to GCC style driver.  This is never an issue for clang because
it manually constructs a --driver-mode option and passes it.

However, we should still try to do as good as we can even if no
--driver-mode is passed.  LibTooling, for example, does not pass
a --driver-mode option and while it could, it seems like we should
still fallback to the best possible default we can.

This is one of two steps necessary to get clang-tidy working on Windows.

Reviewed By: rnk
Differential Revision: https://reviews.llvm.org/D23454

llvm-svn: 278535