[gn] Use std::unordered_map for waiting_map_

Previously, BuilderRecord::waiting_map_ was a base::flat_map. For
heavily depended-upon targets (such as core libraries like //base:base
and popular configs) that have thousands of dependents, inserting a new
dependent in waiting_map_ caused a binary search followed by an O(N)
memmove of all subsequent elements to keep the flat_map sorted.

Once StringAtomSet global lock contention is eliminated, worker threads
parse and submit targets to the Builder at a much higher rate, making
these O(N) memmove insertions a major bottleneck on the main thread.

However, waiting_map_ only requires point lookup by pointer and full
traversals to notify dependents of state changes; it never requires
sorted order (and functions like GetSortedUnresolvedDeps already sort
their output explicitly).

Switching to std::unordered_map eliminates the array shifts on insertion.

Benchmark results (`gn gen out/Default` on Chromium, 5 runs with hyperfine,
official release build configuration with Fuchsia Clang + libc++ +
static libjemalloc.a + LTO):
- Stacked on StringAtomSet sharding (CL 26540):
  2.258s ± 0.035s -> 2.060s ± 0.064s (-198ms / -8.8%, min 2.011s)
- Compared to origin/main:
  2.755s ± 0.029s -> 2.060s ± 0.064s (-695ms / 1.34x faster)

Output ninja files remain bit-for-bit identical across all 14,639 files.

TAG=agy
CONV=4cb5d931-60a8-4f44-8632-3e8d9fb10a8d

Change-Id: I3174ee798a5f6d0c58736fc05ad4ed2684b1d37c
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/26560
Reviewed-by: Matt Stark <msta@google.com>
Commit-Queue: Takuto Ikuta <tikuta@google.com>

src/gn/builder_record.h

diff --git a/src/gn/builder_record.h b/src/gn/builder_record.h
index 52d4816..b2bef55 100644
--- a/src/gn/builder_record.h
+++ b/src/gn/builder_record.h
@@ -6,6 +6,7 @@
 #define TOOLS_GN_BUILDER_RECORD_H_
 
 #include <memory>
+#include <unordered_map>
 #include <utility>
 
 #include "base/containers/flat_map.h"
@@ -327,7 +328,25 @@
     bool wait_validation_defined = false;
     bool wait_validation_resolved = false;
   };
-  base::flat_map<BuilderRecord*, WaitInfo> waiting_map_;
+
+  // Transparent hash and equality functors to allow heterogeneous lookup with
+  // `const BuilderRecord*` (e.g. `this` in GetSortedUnresolvedDeps()) while
+  // keeping mutable `BuilderRecord*` as the map key so dependents can be
+  // modified during notifications.
+  struct PtrHash {
+    using is_transparent = void;
+    size_t operator()(const BuilderRecord* p) const {
+      return std::hash<const BuilderRecord*>()(p);
+    }
+  };
+  struct PtrEqual {
+    using is_transparent = void;
+    bool operator()(const BuilderRecord* a, const BuilderRecord* b) const {
+      return a == b;
+    }
+  };
+
+  std::unordered_map<BuilderRecord*, WaitInfo, PtrHash, PtrEqual> waiting_map_;
 
   // Called by NotifyDependentsOfStateChange() when a dependency
   // of the current record has changed its state. Must return