[llvm][DebugInfo] Backport DW_AT_default_value for template args

**Summary**

Starting with DWARFv5, DW_AT_default_value can be used to indicate
that a template argument has a default value. With this patch LLVM
will emit the this attribute earlier versions of DWARF, unless
compiling with -gstrict-dwarf.

Differential Revision: https://reviews.llvm.org/D139953