Fix the handling of Unicode characters in log and assertion messages

[var-const]

Currently, any UTF-8 characters outside the ascii set may be encoded incorrectly.

[google-oss-bot]

Coverage Report

Affected SDKs

• FirebaseFirestore-iOS-FirebaseFirestore.framework

  SDK overall coverage changed from 88.49% (d50dd0a) to 88.53% (e06d665) by +0.04%.

  Filename	Base (d50dd0a)	Head (e06d665)	Diff
  leveldb_key.cc	96.79%	97.96%	+1.17%
  ordered_code.cc	92.68%	93.24%	+0.56%
  task.cc	95.21%	96.41%	+1.20%
  write_stream.cc	95.65%	93.48%	-2.17%

Test Logs

• Base (d50dd0a5): https://github.com/firebase/firebase-ios-sdk/actions/runs/1016348845
• Head (e06d6656): https://github.com/firebase/firebase-ios-sdk/actions/runs/1016793606