Merging the 6.15.0 release into master (#4682)
Merging the 6.15.0 release into master (#4682)

* Update versions for Release 6.15.0

* Update podspec dependencies for M62 (#4640)

* Cherry-picks for M62. (#4650)

* Include GoogleUtilities in the release manifest.

* Crashlytics fix header import ordering issue (#4649)

Co-authored-by: Sam Edson <8b90bd8605403c4643636c571d09a0f3ddc886c7@google.com>

Co-authored-by: Ryan Wilson <ddab7ce89ad5a4b94dcefca17a5f519bbf3f0954@google.com>

google/GoogleUtilities,google/GoogleUtilities,google/GoogleUtilities,google/GoogleUtilities