[clang][DebugInfo] Revert to attaching DW_AT_const_value on static member declarations (#73626)

In https://github.com/llvm/llvm-project/pull/71780 we started emitting
definitions for all static data-members with constant initialisers, even
if they were constants (i.e., didn't have a location). We also dropped
the DW_AT_const_value from the declaration to [help resolve
inconsistencies during type merging in the
DWARFParallelLinker](https://github.com/llvm/llvm-project/pull/68721).
However, for static data members that do have locations, we wouldn't
emit a DW_AT_const_value on it, assuming that the consumer knows how to
read the value using the location. This broke some consumers that really
wanted to find a DW_AT_const_value. Ultimately we want to attach a
DW_AT_const_value to definitions that have a location too. But to fix
consumers broken by said change, this patch adds the constant back onto
the declaration. This is what we used to do prior to
https://github.com/llvm/llvm-project/pull/71780