Fix a recursion that could happen when creating the first frame in

an unwind because RegisterContextLLDB::InitializeZerothFrame() would
create a minimal stack frame to fetch the pc value of the current
instruction.  This proved fragile when another section of code was
trying to create the first stack frame and UnwindLLDB called
RegisterContextLLDB which tried to create its minimal stack frame.

Instead, get the live RegisterContext, retrieve the pc value from
the registers, and create an Address object from that.

llvm-svn: 151714