bazel build: enable llvm-based demangling (mostly for rust support)

I'm setting the gn var in tools/gen_bazel since it seemed simpler than
trying to get the right expression in gn/perfetto.gni.

Bug: 204297290
Change-Id: Id04793c5cdcff89462bd465e4e7d0eaf8b00124a
diff --git a/BUILD b/BUILD
index 5d93094..42a06c2 100644
--- a/BUILD
+++ b/BUILD
@@ -1353,6 +1353,8 @@
         ":include_perfetto_ext_base_base",
         ":include_perfetto_ext_trace_processor_demangle",
     ],
+    deps = [
+    ] + PERFETTO_CONFIG.deps.llvm_demangle,
     linkstatic = True,
 )