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.
