[lldb] Add dummy field to RegisterInfo for register flags use later

This structure is supposed to be trivial, so we cannot simply do
"= nullptr;" on the new member. Doing that means you are non trivial,
regardless of whether you emulate the previously implied constructor somehow.

The next option is to update every use of brace initialisation.

Given that this is some hundreds of lines, this change just adds a dummy
pointer that is set to nullptr. Subsequent changes will actually use that
to point to register flags information.

Note: This change is not clang-format-ted because it changes a bunch of
areas that are not themselves formatted. It would just add noise.

Reviewed By: jasonmolenda, JDevlieghere

Differential Revision: https://reviews.llvm.org/D145568