When importing a C header in the C++ language mode, Clang/Swift treat C structs as C++ structs.
Currently Swift synthesizes a default initializer that zero-initializes the backing memory of the struct for C structs, but not for C++ structs.
This is causing issues in existing projects that use C libraries and rely on having the default initializer available in Swift. This change enables the synthesis of a default initializer for C++ structs. Since many C++ structs are not designed to be initialized this way, the initializer is marked as deprecated in Swift.
rdar://109727620
