[clang][DebugInfo] DWARFv5: static data members declarations are DW_TAG_variable (#72235)

This patch implements the DWARFv5 issue 161118.1: "DW_TAG for C++ static
data members".

This will simplify LLDB's handling of static data members greatly in the
long term since we no longer need to differentiate non-static from
static data member declarations using non-portable heuristics.

Depends on:
* https://github.com/llvm/llvm-project/pull/72234