Clean up NativeRegisterContext

Summary:
This commit removes the concrete_frame_idx member from
NativeRegisterContext and related functions, which was always set to
zero and never used.

I also change the native thread class to store a NativeRegisterContext
as a unique_ptr (documenting the ownership) and make sure it is always
initialized (most of the code was already blindly dereferencing the
register context pointer, assuming it would always be present -- this
makes its treatment consistent).

Reviewers: eugene, clayborg, krytarowski

Subscribers: aemerson, sdardis, nemanjai, javed.absar, arichardson, kristof.beyls, kbarton, uweigand, alexandreyy, lldb-commits

Differential Revision: https://reviews.llvm.org/D39837

llvm-svn: 317881