Xcode: Inherit global settings in per-target WARNING_CFLAGS (#15224)

Allow projects to use CMAKE_CODE_ATTRIBUTE_WARNING_CFLAGS to add their
own warning flags and have them used by the targets.
Merge topic 'xcode-inherit-warning-flags'

820e95d0 Xcode: Inherit global settings in per-target WARNING_CFLAGS (#15224)