Which is also used by AArch64 and LoongArch.

To test this, I ran the test suite as usual on Arm and found no new 
failures, then ran it again with all test programs compiled with
`-mthumb`. This means the binaries will be entirely Thumb code.

Finally I tested it on AArch64, but this is mostly a build test,
as I did not run the entire test suite compiled to AArch32.

Prior to this change, these tests failed with `-mthumb`:
```
Failed Tests (14):
  lldb-api :: commands/process/reverse-continue/TestReverseContinue.py
  lldb-api :: functionalities/breakpoint/breakpoint_by_line_and_column/TestBreakpointByLineAndColumn.py
  lldb-api :: functionalities/data-formatter/data-formatter-cpp/TestDataFormatterCpp.py
  lldb-api :: functionalities/reverse-execution/TestReverseContinueBreakpoints.py
  lldb-api :: functionalities/reverse-execution/TestReverseContinueWatchpoints.py
  lldb-api :: functionalities/tail_call_frames/disambiguate_call_site/TestDisambiguateCallSite.py
  lldb-api :: functionalities/tail_call_frames/disambiguate_paths_to_common_sink/TestDisambiguatePathsToCommonSink.py
  lldb-api :: functionalities/tail_call_frames/disambiguate_tail_call_seq/TestDisambiguateTailCallSeq.py
  lldb-api :: functionalities/tail_call_frames/inlining_and_tail_calls/TestInliningAndTailCalls.py
  lldb-api :: functionalities/tail_call_frames/sbapi_support/TestTailCallFrameSBAPI.py
  lldb-api :: functionalities/tail_call_frames/thread_step_out_message/TestArtificialFrameStepOutMessage.py
  lldb-api :: functionalities/thread/jump/TestThreadJump.py
  lldb-api :: lang/c/vla/TestVLA.py
  lldb-api :: linux/sepdebugsymlink/TestTargetSymbolsSepDebugSymlink.py
```

After this change, no new failures occurred. So I am confident it is
correct / as bad as it always was.

Looking at those failures, it's a few things:
* Something in the reverse execution wrapper that I can't figure out,
but is
  likely nothing to do with the real breakpoints.
* The inability to tail call when building thumb code, because the call
goes
  via. a veneer that might do a mode switch.
* Assumptions about source locations being in specific places.

None of which are issues I feel like need fixing before doing this port.

I suspect there is redundant code in this, particularly aligning
addresses. I've not made an effort to remove it because A: I'm scared to
break Thumb support because it's not commonly used and B: it may be
there to handle clients other than LLDB, which don't align breakpoint
addresses before requesting them.

Found when building with MSVC on Windows, was seeing:
```
[2703/7138] Building CXX object tools\lldb\source\Plugins\Process\Utility\CMakeFiles\lldbPluginProcessUtility.dir\NativeRegisterContextDBReg.cpp.obj
C:\git\llvm-project\lldb\source\Plugins\Process\Utility\NativeRegisterContextDBReg.cpp(286): warning C4305: 'return': truncation from 'unsigned int' to 'bool'
```

Since the setup of debug registers for AArch64 and LoongArch is similar,
we extracted the shared logic from Class:
`NativeRegisterContextDBReg_arm64`
into a new Class:
`NativeRegisterContextDBReg`.
This will simplify the subsequent implementation of hardware breakpoints
and watchpoints on LoongArch.

Reviewed By: DavidSpickett

Pull Request: https://github.com/llvm/llvm-project/pull/118043