Move the old pass manager infrastructure into a legacy namespace and
give the files a legacy prefix in the right directory. Use forwarding
headers in the old locations to paper over the name change for most
clients during the transitional period.

No functionality changed here! This is just clearing some space to
reduce renaming churn later on with a new system.

Even when the new stuff starts to go in, it is going to be hidden behind
a flag and off-by-default as it is still WIP and under development.

This patch is specifically designed so that very little out-of-tree code
has to change. I'm going to work as hard as I can to keep that the case.
Only direct forward declarations of the PassManager class are impacted
by this change.
[PM] Start sketching out the new module and function pass manager.

This is still just a skeleton. I'm trying to pull together the
experimentation I've done into committable chunks, and this is the first
coherent one. Others will follow in hopefully short order that move this
more toward a useful initial implementation. I still expect the design
to continue evolving in small ways as I work through the different
requirements and features needed here though.

Keep in mind, all of this is off by default.

Currently, this mostly exercises the use of a polymorphic smart pointer
and templates to hide the polymorphism for the pass manager from the
pass implementation. The next step will be more significant, adding the
first framework of analysis support.
[Sanitizers] Share some stack walking code between Windows and Linux

Reviewed at http://llvm-reviews.chandlerc.com/D2126
[arm] Refine ARMBuildAttrs.h.

This commit cleans up some comments in ARMBuildAttrs.h.
Besides, this commit fixes an error related to AllowWMMXv1
and AllowWMMXv2 (although they are not used currently.)
Add support for -fuse-ld=.