Add entries in Modules and Modules/Platform to support
Objective-C++ compiler determination and identification.
Add Modules to check Objective-C++ compiler flags, source
compilations, program checks, etc...
Use OBJCXX as the designator of the language, eg:
project(foo OBJCXX)
Add various tests for Objective-C++ language features. Add
tests to preserve C++ handling of .M and .mm files when
Objective-C++ is not a configured language.
Co-authored-by: Cristian Adam <cristian.adam@gmail.com>
