[clang][DebugInfo] Attach DW_AT_default_value to template arg metadata regardless of DwarfVersion

**Summary**

Starting with DWARFv5, DW_AT_default_value can be used to indicate
that a template argument has a default value. With this patch Clang
will attach the attribute to the debug metadata regardless of version.

In a follow-up patch we will change llvm to emit this attribute in
earlier versions of DWARF, unless compiling with -gstrict-dwarf.

**Details**

* Previously the DwarfVersion check in CGDebugInfo was inconsistent:
  For non-type template arguments we attached the attribute to the debug
  metadata in DWARFv5 only. Whereas for type template arguments we didn't
  have such a version restriction. With this patch we attach the attribute
  regardless of DWARF version (and instead offload the check to the AsmPrinter
  in a future patch).