Args::StringToGenericRegister will now accept "lr" as
another way to indicate that this register is a generic
Return Address register (in addition to "ra") - this is
used primarily by OperatingSystem plugins.

Correctly annotate the UnwindPlan created by EmulateInstructionARM64
to indicate that it was not sourced from a compiler and it
is valid at every instruction.
<rdar://problem/16639754>