Use std::vector for the array of RegisterInfo structs that describe the register context.
- Ensures that this container is populated once for the lifetime of lldb
--- In particular, static methods can query this data even after the first RegisterContext has been destroyed.
- Uses a singleton function to avoid global constructors.

Thanks to Greg Clayton for the suggestion!


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@183313 91177308-0d34-0410-b5e6-96231b3b80d8