[gdb/symtab] Handle interesting_symbols in read_file_scope

Use the interesting_symbols vector to filter the DIEs that we process when
doing read_file_scope.

At this point we start to get the benefit of lazy expansion:
...
$ time gdb -q -iex "maint set lazy-expand-symtab 0" -batch \
    lto/cc1 -ex "b do_rpo_vn"
Breakpoint 1 at 0xd40e30: do_rpo_vn. (2 locations)
real: 5.69
user: 5.47
system: 0.20
...
and with:
...
$ time gdb -q -iex "maint set lazy-expand-symtab 1" -batch \
    lto/cc1 -ex "b do_rpo_vn"
Breakpoint 1 at 0xd40e30: do_rpo_vn. (2 locations)
real: 2.75
user: 2.71
system: 0.10
...

0 files changed, 0 insertions, 0 deletions