[libbacktrace] Unify function name preference handling

[libbacktrace] Unify function name preference handling

Both read_function_entry and read_referenced_name implement a priority scheme
for names.  The priorities are:
- 1st: DW_AT_linkage_name
- 2nd: Name from DW_AT_abstract_origin or DW_AT_specification
- 3rd: DW_AT_name.

Ensure both functions fully adhere to it.

2019-01-16  Tom de Vries  <tdevries@suse.de>

	* dwarf.c (read_referenced_name): Don't allow DW_AT_name to override any
	name.
	(read_function_entry): Same.  Don't allow name found via
	DW_AT_abstract_origin or case DW_AT_specification to override linkage
	name.

From-SVN: r267963