All UnwindPlan objects are now passed around as shared pointers.

ArchDefaultUnwindPlan plug-in interfaces are now cached per architecture 
instead of being leaked for every frame.

Split the ArchDefaultUnwindPlan_x86 into ArchDefaultUnwindPlan_x86_64 and
ArchDefaultUnwindPlan_i386 interfaces.

There were sporadic crashes that were due to something leaking or being 
destroyed when doing stack crawls. This patch should clear up these issues.

llvm-svn: 125541