Fix use of "main" marker in gdb index

Fix use of "main" marker in gdb index

Tom de Vries noticed that with .gdb_index, the "main" marker would
sometimes seemingly be ignored.

I tracked this down to an interaction between the rewritten reader and
the "main"-finding code in cooked_index.  With the ordinary DWARF
scanner, a C "main" won't be marked as IS_MAIN; whereas with
.gdb_index this can happen.  In this case, the code thinks that C
requires canonicalization (which is only true for types), and skips
using the symbol.

This patch fixes the problem and adds some comments explaining what is
going on.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33441