Add "synthetic" marker for index entries

Currently, gdb will synthesize DW_TAG_module entries for Ada names.
These entries are treated specially by the index writer,

When GNAT starts emitting DW_TAG_module, the special case will be
incorrect, because there will be non-synthetic DW_TAG_module entries
in the index.

This patch arranges to mark the synthetic entries and changes the
index writer to follow.