[gn build] Link with -Wl,--gdb-index when linking with LLD

For full-debug-info (is_debug=true / symbol_level=2 builds), this makes
linking 15% slower, but gdb startup 1500% faster (for lld: link time
3.9s->4.4s, gdb load time >30s->2s).

For link time, I ran

    bench.py -o {noindex,index}.txt \
        sh -c 'rm out/gn/bin/lld && ninja -C out/gn lld'

and then `ministat noindex.txt index.txt`:

```
x noindex.txt
+ index.txt
    N           Min           Max        Median           Avg        Stddev
x   5      3.784461     4.0200169     3.8452811     3.8754988   0.089902595
+   5       4.32496     4.6058481     4.3361208     4.4141198    0.12288267
Difference at 95.0% confidence
	0.538621 +/- 0.15702
	13.8981% +/- 4.05161%
	(Student's t, pooled s = 0.107663)
```

For gdb load time I loaded the crash in PR48392 with

    gdb -ex r --args ../out/gn/bin/ld64.lld.darwinnew @response.txt

and just stopped the time until the crash got displayed with a stopwatch
a few times. So the speedup there is less precise, but it's so
pronounced that that's ok (loads ~instantly with the patch, takes a very
long time without it).

Only doing this for LLD because I haven't tried it with other linkers.

Differential Revision: https://reviews.llvm.org/D92844

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