Prefix operators in Swift need to be marked as `prefix func`.
For example, the lack of `prefix` attribute prevents the user from conforming a C++ type that defines `operator!` to a Swift protocol that requires `static prefix func !(obj: Self) -> Self`.
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.
