value_util.cc: Fix MaxValue() to no longer use designated initialization.

This ports firebase/firebase-ios-sdk#9868 to the firebase-cpp-sdk repository by patching the firebase-ios-sdk when it is cloned.

Once the dependency on the firebase-ios-sdk is updated to a version that already includes firebase/firebase-ios-sdk#9868 then this commit should be reverted.

Author: dconeybe

cmake/external/firestore_snappy.patch.txt

@@ -617,3 +617,68 @@ index 000000000..28bfb0837
 +   const uint8_t*& ip = *ip_p;
 +   // This section is crucial for the throughput of the decompression loop.
 +   // The latency of an iteration is fundamentally constrained by the
+diff --git a/Firestore/core/src/model/value_util.cc b/Firestore/core/src/model/value_util.cc
+index 1b6bf07e0..41d1d6d26 100644
+--- a/Firestore/core/src/model/value_util.cc
++++ b/Firestore/core/src/model/value_util.cc
+@@ -53,26 +53,6 @@ const char* kRawMaxValueFieldValue = "__max__";
+ pb_bytes_array_s* kMaxValueFieldValue =
+     nanopb::MakeBytesArray(kRawMaxValueFieldValue);
+ 
+-/** The special map field value entry of a maximum proto value. */
+-google_firestore_v1_MapValue_FieldsEntry kMaxValueFieldEntry = {
+-    .key = kMaxValueFieldKey,
+-    .value = {
+-        .which_value_type = google_firestore_v1_Value_string_value_tag,
+-        .string_value = const_cast<pb_bytes_array_t*>(kMaxValueFieldValue)}};
+-
+-/** The special map value of a maximum proto value. */
+-_google_firestore_v1_MapValue kMaxValueMapValue = {
+-    .fields_count = 1, .fields = &kMaxValueFieldEntry};
+-
+-/**
+- * A maximum value that is larger than any other Firestore values. Underlying it
+- * is a map value with a special map field that SDK user cannot possibly
+- * construct.
+- */
+-google_firestore_v1_Value kMaxValue = {
+-    .which_value_type = google_firestore_v1_Value_map_value_tag,
+-    .map_value = kMaxValueMapValue};
+-
+ }  // namespace
+ 
+ using nanopb::Message;
+@@ -703,8 +683,32 @@ bool IsMinValue(const google_firestore_v1_Value& value) {
+   return IsNullValue(value);
+ }
+ 
++/**
++ * Creates and returns a maximum value that is larger than any other Firestore
++ * values. Underlying it is a map value with a special map field that SDK user
++ * cannot possibly construct.
++ */
+ google_firestore_v1_Value MaxValue() {
+-  return kMaxValue;
++  google_firestore_v1_Value value;
++  value.which_value_type = google_firestore_v1_Value_string_value_tag;
++  value.string_value = kMaxValueFieldValue;
++
++  // Make `field_entry` static so that it has a memory address that outlives
++  // this function's scope; otherwise, using its address in the `map_value`
++  // variable below would be invalid by the time the caller accessed it.
++  static google_firestore_v1_MapValue_FieldsEntry field_entry;
++  field_entry.key = kMaxValueFieldKey;
++  field_entry.value = value;
++
++  google_firestore_v1_MapValue map_value;
++  map_value.fields_count = 1;
++  map_value.fields = &field_entry;
++
++  google_firestore_v1_Value max_value;
++  max_value.which_value_type = google_firestore_v1_Value_map_value_tag;
++  max_value.map_value = map_value;
++
++  return max_value;
+ }
+ 
+ bool IsMaxValue(const google_firestore_v1_Value& value) {