[DebugInfo] Remove `dbg.addr` from CodeGen

As part of this work, removing `SDDbgValue::clearIsEmitted` originally added for
`dbg.addr` in 045c6776 was attempted, but it
appears some tests for `DBG_INSTR_REF` now depend on that behaviour as well, so
it was kept and comments were updated instead.

Part of `dbg.addr` removal
Discussed in https://discourse.llvm.org/t/what-is-the-status-of-dbg-addr/62898

Differential Revision: https://reviews.llvm.org/D144800