||DW_IDX_* for static/extern symbols
Section 18.104.22.168.7, pg 147
When a debugger wants to print 'somename' it logically tries to find first 'somename' as an
external symbol in all available libraries. Only if none such external symbol is found the
debugger starts searching for a static 'somename' symbol in those libraries.
This requires to know whether a symbol in .debug_names index has DW_AT_external or not.
Otherwise a lot of needless CU expansions happen. This extension improves performance
gain of the .debug_names index.
(Discovered in an original fix by Doug Evans - GDB Bug 14125.)
Proposing and asking for pre-allocation:
DW_IDX_static = 6 = DW_FORM_flag_present = DIE's DW_AT_external is not present
DW_IDX_external = 7 = DW_FORM_flag_present = DIE's DW_AT_external is present