This adds a new implementation of virtual method dispatch that handles reference types correctly.
Previously, for all C++ types an invocation of a virtual method would actually get dispatched statically. For value types this is expected and matches what C++ does because of slicing. For reference types, however, this is incorrect, we should do dynamic dispatch.
rdar://123852577
This allows calling a C++ function with default arguments from Swift without having to explicitly specify the values of all arguments.
rdar://103975014
C++ `T& operator*()` is mapped to a Swift computed property `var pointee: T`.
Previously `var pointee` only had a getter, after this change it will also have a setter if the C++ type declares an overload of `operator*` that returns a mutable reference.
rdar://112471779
C++ pre-increment operator `T& T::operator++()` is mapped into a non-mutating function `successor() -> Self`.
The naming matches existing functions for `UnsafePointer`/`UnsafeMutablePointer`.
The purpose of this is to be used for iterator bridging: C++ requires iterators to define a pre-increment operator (https://en.cppreference.com/w/cpp/named_req/Iterator), which Swift will use to iterate over C++ sequences and collections.
`ImportDecl.cpp` contained 10k+ lines of code, which caused slowdowns in incremental compilation and while editing the code in the IDE.
This change extracts a chunk of largely self-contained decl synthesis logic into a separate file.
