Hoist the RegisterNumber class out of RegisterContextLLDB and make

it more generally available. 

Add checks to UnwindAssembly_x86::AugmentUnwindPlanFromCallSite() so
that it won't try to augment an UnwindPlan that already describes
the function epilogue.

Add a test case for backtracing out of _sigtramp on Darwin systems.
This could probably be adapted to test the same thing on linux/bsd but 
the function names of sigtramp and kill are probably platform
specific and I'm not sure what they should be.

llvm-svn: 225578