Migrate std::unordered_map -> absl::flat_hash_map.

BUILD

@@ -207,6 +207,7 @@ cc_library(
     copts = CPPOPTS,
     deps = [
         "@absl//absl/base:core_headers",
+        "@absl//absl/container:flat_hash_map",
         "@absl//absl/strings",
         "@com_google_protobuf//:protobuf",
         "@com_google_protobuf//:protoc_lib",

upbc/generator.cc

@@ -1,8 +1,7 @@
 
-#include <unordered_map>
-#include <unordered_set>
 #include <memory>
 
+#include "absl/container/flat_hash_map.h"
 #include "absl/strings/ascii.h"
 #include "absl/strings/str_replace.h"
 #include "absl/strings/substitute.h"
@@ -594,7 +593,7 @@ void WriteSource(const protobuf::FileDescriptor* file, Output& output) {
     std::string fields_array_ref = "NULL";
     std::string submsgs_array_ref = "NULL";
     std::string oneofs_array_ref = "NULL";
-    std::unordered_map<const protobuf::Descriptor*, int> submsg_indexes;
+    absl::flat_hash_map<const protobuf::Descriptor*, int> submsg_indexes;
     MessageLayout layout(message);
     std::vector<const protobuf::FieldDescriptor*> sorted_submsgs =
         SortedSubmessages(message);

upbc/message_layout.h

@@ -2,8 +2,8 @@
 #ifndef UPBC_MESSAGE_LAYOUT_H
 #define UPBC_MESSAGE_LAYOUT_H
 
-#include <unordered_map>
 #include "absl/base/macros.h"
+#include "absl/container/flat_hash_map.h"
 #include "google/protobuf/descriptor.h"
 
 namespace upbc {
@@ -70,7 +70,7 @@ class MessageLayout {
   Size Place(SizeAndAlign size_and_align);
 
   template <class K, class V>
-  static V GetMapValue(const std::unordered_map<K, V>& map, K key) {
+  static V GetMapValue(const absl::flat_hash_map<K, V>& map, K key) {
     auto iter = map.find(key);
     if (iter == map.end()) {
       fprintf(stderr, "No value for field.\n");
@@ -92,11 +92,11 @@ class MessageLayout {
   static int64_t FieldLayoutRank(
       const google::protobuf::FieldDescriptor* field);
 
-  std::unordered_map<const google::protobuf::FieldDescriptor*, Size>
+  absl::flat_hash_map<const google::protobuf::FieldDescriptor*, Size>
       field_offsets_;
-  std::unordered_map<const google::protobuf::FieldDescriptor*, int>
+  absl::flat_hash_map<const google::protobuf::FieldDescriptor*, int>
       hasbit_indexes_;
-  std::unordered_map<const google::protobuf::OneofDescriptor*, Size>
+  absl::flat_hash_map<const google::protobuf::OneofDescriptor*, Size>
       oneof_case_offsets_;
   Size maxalign_;
   Size size_;