[Driver] Move architecture-specific free helper functions to their own files.

This patch moves helper functions that are CPU-specific out of Driver.cpp and to
separate implementation files. The new files are named for the architecture,
e.g. ARMArch.cpp.

The next step after this will be to move OS-specific code, which I expect will
include many of the tool implementations, to similarly separate files.

Some CPU-specific functions are not being moved just yet. In cases where the
only caller is the platform-specific tools, I plan to move them together. An
example is Hexagon, where the only caller of the architecture-specific functions
are the tools themselves. (I'm happy to revise this choice, it just seems like
less churn to me.)

This does mean that some functions which were previously static are now exposed
through the library header Driver.h.

Reviewers: rsmith, javed.absar

Subscribers: aemerson, danalbert, srhines, dschuff, jyknight, nemanjai, mgorny, cfe-commits

Differential Revision: https://reviews.llvm.org/D30315

llvm-svn: 296056