[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

utils/gn/build/BUILD.gn

@@ -77,8 +77,25 @@ config("compiler_defaults") {
   if (host_os != "win") {
     if (symbol_level == 2) {
       cflags += [ "-g" ]
+
+      # For full debug-info -g builds, --gdb-index makes links ~15% slower, and
+      # gdb symbol reading time 1500% faster (lld links in 4.4 instead of 3.9s,
+      # and gdb loads and runs it in 2s instead of in 30s).  It's likely that
+      # people doing symbol_level=2 want to run a debugger (since
+      # symbol_level=2 isn't the default). So this seems like the right
+      # tradeoff.
+      if (host_os != "mac" && use_lld) {
+        cflags += [ "-ggnu-pubnames" ]  # PR34820
+        ldflags += [ "-Wl,--gdb-index" ]
+      }
     } else if (symbol_level == 1) {
       cflags += [ "-g1" ]
+
+      # For linetable-only -g1 builds, --gdb-index makes links ~8% slower, but
+      # links are 4x faster than -g builds so it's a fairly small absolute cost.
+      # On the other hand, gdb startup is well below 1s with and without the
+      # index, and people using -g1 likely don't use a debugger. So don't use
+      # the flag here.
     }
     if (is_optimized) {
       cflags += [ "-O3" ]