Merge pull request #12834 from muxi/backport-12772

Backport #12772: Fix Xcode warning of mis-aligned structs

gRPC-Core.podspec

@@ -84,6 +84,7 @@ Pod::Spec.new do |s|
     # build.
     'USE_HEADERMAP' => 'NO',
     'ALWAYS_SEARCH_USER_PATHS' => 'NO',
+    'GCC_PREPROCESSOR_DEFINITIONS' => '"$(inherited)" "COCOAPODS=1" "PB_NO_PACKED_STRUCTS=1"',
   }
 
   s.default_subspecs = 'Interface', 'Implementation'

templates/gRPC-Core.podspec.template

@@ -119,6 +119,7 @@
       # build.
       'USE_HEADERMAP' => 'NO',
       'ALWAYS_SEARCH_USER_PATHS' => 'NO',
+      'GCC_PREPROCESSOR_DEFINITIONS' => '"$(inherited)" "COCOAPODS=1" "PB_NO_PACKED_STRUCTS=1"',
     }
 
     s.default_subspecs = 'Interface', 'Implementation'