Generate DILocalVariable entries for parameters of extern functions,
the "annotations" field of DILocalVariable is used to link
"btf_decl_tag" annotation with the parameter.
Do this only for BPF backend as there are no other users for this
information. Final DWARF is valid as "Appendix A" is very much lax in
what is allowed as attributes for "DW_TAG_formal_parameter":
DWARF does not in general require that a given debugging information
entry contain a particular attribute or set of attributes. Instead,
a DWARF producer is free to generate any, all, or none of the
attributes ... other attributes ... may also appear in a given
debugging information entry.
DWARF Debugging Information Format Version 5,
Appendix A: Attributes by Tag Value (Informative)
Page 251, Line 3.
Depends on D140969