When Swift imports C structs, it synthesizes an initializer that takes no arguments and zero-initializes the C struct.
When C++ interop is enabled, Clang treats all C structs as if they were C++ structs. This means that some of the C structs will get a default constructor implicitly generated by Clang. This implicit default constructor will not zero-initialize trivial fields of the struct.
This is a common source of confusion and subtle bugs for developers who try to enable C++ interop in existing projects that use C interop and rely on zero-initialization of C structs.
rdar://115909532
When a default constructor is declared, but does not have a body because it is defaulted (`= default;`), Swift did not emit the IR for it. This was causing linker error for types such as `std::map` in libstdc++ when someone tried to initialize such types from Swift.
rdar://110638499 / resolves https://github.com/apple/swift/issues/61412
