Implement RegisterContext::WriteRegisterBytes in RegisterContextLLDB.

I only did a tiny bit of testing; in the one case I tried changing the
contents of a radar in the middle of a stack and it was still current in
the live register context so it filtered down to frame 0 and was handed
over to the live register set RegisterContext.  I need to test a case
where a register is saved on the stack in memory before I check this
one off.

llvm-svn: 118486