[lldb] [gdb-remote] Refactor getting remote regs to use local vector

Refactor remote register getters to collect them into a local
std::vector rather than adding them straight into DynamicRegisterInfo.
The purpose of this change is to lay groundwork for switching value_regs
and invalidate_regs to use local LLDB register numbers rather than
remote numbers.

Differential Revision: https://reviews.llvm.org/D110025